There's a difference in "doing it as a wedding present" (for friends/family) and "doing it return for money". The "must have"s stay the same, but I would expect a professional to cover most of the "should have"s too.
Having the right equipment
There are three main groups of kit, the "must", the "should" and the "nice to":
- Lens (Something like a standard or wide zoom would work; I've seen a lot of pros using Canon's 17-40/f4)
- Spare batteries
- About 4 times as many memory cards as you think you'll need
- Spare camera
- Long lens (this is a stylistic point though, but good at getting natural shots of people enjoying themselves without being too "in your face")
- Neutral (and without pattern) golf umbrella just in case it rains.
- Separate flash (ideally one that can be angled and rotated to bounce off convenient surfaces
- Step stool/ladder to get that little bit of extra height.
Nice to have
- Eyes in the back of your head (or a second photographer) to pick up on some of the more informal details that the bride and groom might appreciate.
- Some kind of portable bench to provide somewhere for the bride and groom to sit and sip their champagne, whilst having their photo taken (etc).
Regarding crowd control
If there are ushers, make them earn their keep and try and get them to assist -- they generally will know who is family, who are friends, and whether they're on the bride or groom's side, and can usually be coaxed into helping (even if it's a gentle nudge along the lines of "the sooner you help get these done, the sooner the bar can prop you up")
Be assertive; Generally, guests are used to being corralled at weddings and are understanding that you're just doing you job.
Make sure you look around and pay attention to those that aren't aware of the camera - for example, children picking up confetti or the ushers chatting up the bridesmaids...