Yes, everything Woody said. Practice before hand. Practice with sun behind the subject, sun in their eyes, indoors. Practice in good and bad light. For my daughter, it was a struggle. Her dress was blue, so I was thankful for that. We met at the river for pictures very late afternoon. I even went there before hand to practice. On Prom night it was packed solid. So we pretty much followed the crowd to all the worst spots. Blazing sun over their heads, blazing sun in their eyes. I did manage to get some good pictures though as the crowd thinned and I could position better. Even the poorly positioned shots were not too bad. I used fill flash on some and kept the sun out of the viewfinder. Definitely shoot raw + jpg. Even if you have no idea about raw, you can learn what to do with the raw if needed. If there is a boy in black involved, it will be even more tricky to show the texture in the tux as well as not blow out the dress. Raw will help with that. Practice changing that exposure compensation quickly.