I spoke with a colleague of mine who photographs semi-professionally on the side. Here's what advice he had to offer...
Gear Related –
Everyone differs depending on shooting style so there are no must have combos. I shoot with a 24-105 zoom which covers a nice range on a dSLR with a 35mm prime on a second body. If allowed, flash is very helpful as well and a tripod may be useful (again, depends on your style and what is allowed at the event).
If you are using flash, make sure you have some sort of powerpack that will allow the flash to recycle quickly.
Definitely have backups of everything from camera body, lens, and memory cards in the event something fails. I recommend using more and smaller memory cards (say 2GB). You won’t have much time to do backup so the more the merrier.
Non gear related (more important than gear) –
Practice practice practice and plan very well before the event occurs. Blowing the hilite’s from the bride’s white dress or underexposing the dark tux will be bad news so practice as much as possible in the same setting as the wedding if it is possible to simulate.
Disorganization is an event killer and will make you look unprofessional, even when not really your fault. At a wedding I shot, everything was tossed together last minute and the bride didn’t know what family shots she wanted…they looked to me to tell them at the time allotted for this activity which was a near disaster. This was a great learning experience for me…it was my job to make sure this was ironed out before the big event which I didn’t do.
People skills are very important…don’t just stumble around shooting people at random. Talking to the guests and wedding party helps a great way in easing moods and getting better smiles.
Anticipation is also very important but that is stating the obvious.
www.stellaryear.com: Canon EOS 50D, Canon EOS 5DMkII, Sigma 50mm 1.4, Canon 24-70 2.8L II, Canon 100mm 2.8L, Canon 100-4