This Friday I have my first paid gig, shooting portraits for some family friends. I am going to take the photos around 5, in a local Arboretum. Are there any particular characteristics that I should look for in locations? Also, are there any other tips that I should know? I will be shooting with a Canon 50d and 18-55mm f/3.5 75-300mm f/4.0 and 50mm f/1.8 lenses.
It may be a bit late for this, but be sure to check whether the arboretum has any photo policies. I know that in two locations I looked into here, one prohibits commercial photography unless the photographer purchases a license. The other was fine with small groups, but they've got some guidelines about behavior and the size of your entourage (ex: stay on posted trails). Remember, these organizations exist for the enjoyment of lots of other people, too, and they'll manage activity accordingly.
Speaking of other people, you'll probably want to pay attention to other guests wandering through your backgrounds if you're not right next to a backdrop. If you've not already visited the location to scout out locations, it would be really beneficial to sneak in a trip ahead of time. Pay attention to walls, bridges, streams, arbors, etc. -- all of these can be interesting, provided they're in areas you're allowed to go.
Watch your lighting, if at all possible, try to conform to three point lighting with a key, fill and back light. If you have multiple strobes, use them. If you can find area's with white ceilings that aren't too too far up, great, use them to bounce the flashes. The biggest key with good portraits is going to be proper focal length, good background blur and proper lighting. (Also, positioning them well in front of whatever background will be key.)