Okay, I'd like to get to know more about photography, but not in a super serious way. In particular, I'd like to find a few photographers that I could offer to be second shoot, so I can improve my skills. Ideally, I'd work an arrangement where I could use my best photos in a portfolio, but nothing else, and give the main photographer all of my prints. If they used any of them then I get some cash, otherwise, I get little/nothing. How could I go about such a thing?
How to Find a Second-Shooter Gig
(A Brief Primer to the Somewhat harsh Realities of the Wedding Photography Industry)
No, Really... How do I Find a Gig?
The most reliable way to find high-quality second-shooter gigs is to research the photographers in your area, check out their websites, and directly contact any photographers whose work you like to give them your information and let them know you're available. It is very common for wedding photographers to ask if you want to be an assistant and work your way into a second-shooter position. The right answer to this questions is: "Absolutely! What do you need?" Be prepared to work a wedding or two for free doing the grunt work tasks (toting gear, getting coffee, assembling light stands, getting coffee, holding reflectors, getting coffee, herding family, etc.) in order for the photographer to 'build trust in you.'
This is a somewhat shady, but very common practice in the wedding industry and I generally advise young photographers to simply 'go with it'... but only for a wedding or two with any given wedding photographer. If you haven't seen a 'promotion' to second-shooter, or some indication that one is coming your way after a wedding or two, you're probably simply being used as free labor and it might be time to have a frank conversation, or cut your losses. :-/ The rule of thumb whether assisting, or being a second-shooter is 'be the first to get there, last to leave, hardest working Johnny on-the-spot guy (or gal) in the operation.' That's what gets you to the top of a photographers list of 'people to call' when they need help. Keep your ears and eyes open and your mouth shut and you can find out a ton about what to do (and what not to do) when you become a wedding photographer.
Never use Craigslist to find second-shooter gigs. Almost without exception the folks who advertise for wedding photography on Craigslist are the ones who do low-quality, high-volume work. There will be very little (if anything) that you'll be able to learn about the craft of wedding photography from them (very often they are just learning themselves and have nothing to teach) and most of the time they are the sort of people who work without contracts (see the last part of this answer for why this is terrible for you)...
What to Expect from the Legit Gigs you Do Find
With that out of the way... In my experience (and from the lead photographers point-of-view), 'second shooter gigs' tend to come in one of three flavors:
Flat fee: The lead photographer pays the second shooter a set rate for their services. Most of the time flat fee second shooters are in a work-for-hire arrangement, and you can expect that (unless the photographer is a noob, or unusually generous) the photographer will insist that you perform your work in a work-for-hire arrangement. In other words, they get the pictures you take, can claim them as their own, and you cannot use them for your portfolio. This is by far the most common arrangement for second shooters in the wedding industry, though more-often-than-not it is only offered to 'trusted associates' and people who have proven to the lead photographer that they can take 'client ready' photographs. It would be very rare to be offered this sort of arrangement in your first gig with a photographer unless you already come packin' a pretty epic portfolio of your own.
No fee: The lead photographer doesn't pay a second shooter, but they do get to use any pictures they take as portfolio pieces. This is almost never an option unless the lead photographer is fairly new and building his or her own portfolio, or is unusually generous. This is because most working wedding photographers who aren't new would much rather pay someone a flat fee so they don't ever end up having to explain to potential clients why pictures from the wedding they 'supposedly' shot are also on someone elses website. In essence it is only someone who has either under-quoted their fee to their client (and as such cannot afford to pay a second shooter), or is new to the industry (and thus doesn't have enough clout or enough contacts to have a 'stable' of second shooters the use on a regular basis) who willingly chooses a 'no fee' arrangement.
Apprentice: the lead photographer doesn't pay the second shooter (in fact, often the second shooter pays a fee to the lead photographer), and the second shooter is in a work-for-hire arrangement, so they don't get to keep any of the images nor use them in their portfolio. Why would anyone willingly choose this option when it seems like the 'worst of both the above options?' Generally this is the option that you end up with if you are absolutely new (e.g. easy to take advantage of because you've got nothing the lead photographer needs), or if you're more experienced but the lead photographer is very well known and/or the wedding is a large one, or for famous people... The assumption in this situation is that what you get out of the deal is the experience of tailing a well-known photographer, or the experience of shooting a wedding you may not otherwise be qualified to shoot... This arrangement is often the 'norm' in big cities, where photographers looking to break into wedding photography are a dime-a-dozen.
But What's in it for Me???
Now you'll notice that none of these arrangements (in general) favor the second shooter. Yep. Welcome to the bottom rung of the food chain (Well, second to the bottom... The assistants are below you) in the wedding photography industry. 'How to build your wedding photography portfolio' is another question entirely, but (again, generally speaking) most of the time 'go be a second shooter' is not the way to do that. The above arrangements do offer you some value: experience, industry contacts, and sometimes cash, which is often the best deal to be made with second shooting. A common sentiment that often gets expressed by wedding photographers is 'go build your portfolio on your own dime, not mine.'
Once you've been in the wedding photography business a while and you're seen more as a peer than a lackey the treatment is often better... Second shooting is done more as a 'favor/courtesy' for fellow photographers and they 'owe you one' and will (often) help you out when you need a second and they're available. Depending on where you live, if the professionals in the area are well organized and work together frequently this may make it really tough to 'break in' because why would someone want to take a chance on an untested noob, when they can just call in a pro who's work they already know and who (more importantly) already has a portfolio of their own so they won't come bugging them to 'just use that one shot I took that is soooooo awesome for my portfolio and website pleeeeeese?!'
My only advice in these kinds of markets is to check in periodically with photographers (don't be a pest, though), and be patient until you get that first call... Then when you do get that call, work your @ss off like you've never worked before. Work hard and produce good pictures and word will get around. If you make someone look good and their business more successful, you'll get more calls. Conversely, if you make the lead photographer look bad, you force him/her to babysit you, you're generally annoying, a pain in the @ss, or you are sub-par in any way (regardless of whether it was your fault or not... Excuses don't play) you can expect to never get called again at best, or blacklisted at worst. Hey, it's a harsh world out there. If you wanted it easy you should have stayed in college and become an accountant like your momma told you to. :-)
Danger, Will Robinson!
Whatever type of second-shooting situation arrangement you find yourself in, insist on a contract! Unless the photographer is brand-new (or not especially bright), they will have one... In fact, I'd be wary if any photographer who doesn't bring it up with you first. :-) This will clearly define your responsibilities and the expectations that the primary shooter has for you, and will (if it's written correctly) indemnify you from a whole host of 'bad possibilities' that could happen as a result of working as a second shooter. Never work without it.
One of my photographer friends found himself on the receiving end of a lawsuit on the very first wedding he ever shot (as a second shooter) when the primary photographer skipped town without delivering a couple's wedding photographs. Since they couldn't find the primary photographer, who do you think got served with the papers? You guessed it. He weathered the storm and has been successfully shooting weddings for more than 5 years now, but all of that could have been avoided had he insisted on a contract (and the primary photographers shady nature probably would have been brought out up front, as he wasn't using any contracts)...
A Final Note...
As with anything in life, your mileage may vary. If you live and work outside of the US, none of the above may apply to you at all, as the only experience I have is in the American wedding industry. Generally speaking in bigger American communities the above will hold 'more true' than in smaller ones (if anything some areas of the US can be harsher than I've described... It can be an absolute soul-sucking bloodbath starting out in mega-big cities like Los Angeles, Chicago, Boston, and New York... 'If you can make it there you can make it... bla, bla, bla...') Conversely, if you live in a smaller community, or in a community with a photographer (or group of photographers) who really believes in teaching and training people to be better photographers, the atmosphere can be downright genteel. Having said all of this I'll leave it with this one final thought (also from personal experience)... If you think the wedding photography industry is harsh, you should try breaking in to Hollywood! Yikes. Now there is a chum filled shark-tank for you... Be grateful you've chosen an easy career-field to enter. ;-)
Here's my perspective as a pro photographer who occasionally hires someone to second shoot with me. There are two main factors in choosing an second shooter. They are:
How do I connect with other potential shooters? Craigslist is one option (mentioned by another poster) but I find I meet most them by networking with local photography groups. In my area there are a couple groups associated with professional photography organizations such as the PPA and ASMP, a group that are all Flickr users and have a local Flickr meetup, and an informal group that gets together for a photographer's happy hour once a month. I've met lots of great photographers and made some excellent connections at these offline networking events that really lets me get to know people and works out better than just finding someone on Craiglist.
Craigslist is how I found photographers to shoot with, I posted that I was looking to be an assistant and responded to ads for assistants. That and forums like fredmiranda.com and dgrin.com, people post 'looking for assistants' there often.
Never tried that myself, but I would suggest sending them a link to your portfolio of relevant images and asking whether you can be their second shooter.