I’m not a pro, though I’ve shot more weddings than I’ve ever wanted as favors for friends. Some things I’ve learned are:

1. Figure out in advance when you’re going to take the posed shots and make sure that time is solidly allocated and your subjects aren’t going to be agitated because they want/need to run to something else.
2. Ask the client up front what shots they want to make sure you don’t miss something they specifically want. Plan out in your head what’s coming next – the kiss, the cake cut, etc. so that you’re not fumbling with battery or media changes during a critical moment. Fire like crazy when those shots come up because it increases your chance of a good hit.
3. Put a wide lens on one camera and a long one on the other. You need to be ready for any situation. That also allows you to swap the lenses if needed.
4. Check with both the pastor/priest/etc. and the client before the wedding to find out what you’re allowed to do during the ceremony. Nothing rots worse than to find out that you’re not allowed to use flash or have them annoyed because you got too close to the pulpit. Find out and be prepared (incidentally exactly both of these have happened to me).
5. Check out the lighting situation before the wedding to make sure you know how you’ll handle the light or lack thereof.
6. Be prepared to be treated like “the help”. You may be the bride/groom’s friend, but most people probably won’t know that. Usually someone thinks you’re getting paid and treats you accordingly.
7. MAKE SURE YOUR CAMERA CLOCKS ARE SYNCHRONIZED! If not, it can be a lot of work to fix and totally breaks the image sorting later.
8. Be prepared for a lot of post-processing fun to try to match up a images if you’re shooting with two different makes of cameras. The sensor output (“look”) can be significantly different between models/makes and placing them in the same album can be problematic.
9. Batteries, batteries, batteries! I suggest using rechargeable because they recycle much faster, but regardless, make sure you have enough of them and if rechargeable, a plan on how to recharge them.
10. Memory – it goes fast. Make sure you have enough. Consider getting a portable backup hard drive unit. Personally I back up to two separate ones to make sure that if one chooses to die, I have a second copy.
11. Figure out where you’re going to station you equipment. You’re going to potentially have a lot of expensive stuff and it can either be in the way, not convenient, or causing you to be distracted because you’re worrying about theft. Figuring it out on the fly can be a major distraction.
12. As soon as you’ve downloaded the images, back them up securely to additional media. Nothing can cause a group of people to hate you more than to screw this up. Remember, this is someone’s wedding, you get it wrong and your name is “Mud” (and/or will be known as the person who “ruined” their wedding).
13. Make sure you have good flashes. You’re going to need the output.
14. Don’t practice high ISO shots for the first time during a wedding. Make sure you understand the pluses and minuses and what works and doesn’t work. High ISO can be a savior, but particularly in high contrast shots, it can look ugly as all get out. It also is difficult sometimes to match to other shots in any album.
15. Take many multiples of group shots, frankly any posed shots. The LCD preview is a good start, but too small to catch all the possible ways your subjects can screw up an image. Those eyes may look open, but they still look ugly and off.
16. Constantly check that you haven’t bumped or accidentally changed your settings. Nothing sucks more than to realize you’ve been set at maximum aperture with no depth of field because you didn’t realize you knocked the command dial. ALWAYS keep reminding yourself to set back any custom changes like exposure adjustments or ISO changes. It bites to realize you’ve been a half stop down on every picture 200 shots later. This leads to…
17. Shoot RAW. You have a lot more options and hope if things are off or you screw up. JPEGs do not have a lot of latitude. Of course if you shoot RAW, plan a weekend worth of adjustments and processing!
18. Test all your equipment well before and the day of. It really sucks to find you’re one flash down 20 minutes before the wedding. A stupid little forgotten cable can throw a huge wrench in your plans (or mounting plate, as was my case).
19. Figure out where you’re going to take the posed shots and what poses you want. You look like an amateur if you’re doing this on the fly.
20. Act confident and your subjects will believe you actually know what you’re doing. Don’t act confident, and things can get out of hand.
21. Make sure your deodorant is up to snuff, you’re going to sweat running around in a suit.
22. Figure out as early as possible who’s with the bride, who’s with the groom, who’s the mother, the father, the best man, etc. etc. You’re going to need this information to properly case the shots. Doing this on the fly risks error and even possibly insulting someone and can certainly make you look like an amateur.
23. Make sure you don’t favor one family or another in your shots. At the reception take advantage of the seating to make sure you get just about everyone. That way they won’t complain that precious old Uncle Arnold got missed or have one of the families complaining that they were ignored.
24. Have a sense of humor. You’re going to need it. It also helps lighten the mood and get people to smile, which of course is half the battle.

I learned many of these the hard way, which is why you couldn’t pay me to do weddings!

Unfortunately people don’t, but I do them anyway…