My main tip would be to practice it quite a bit first. My sister did a Maid of Honor speech for her best friend. She left it to the last minute (not her fault, she has a very full on job and no spare time at all!) and although I tried to help her rehearse the night before she was still reading far too fast, not nearly loud enough and rolling her eyes at the ceiling periodically, when it was time for her to go. I wasn't at the wedding so not sure how it went but as with any speech my pointers would be:
Keep it short and sweet - 5 minutes worth maybe
If you think you can, take note cards to prompt you with a speech you've memorized rather than properly reading from a paper
Any humorous stories would be great, as long as they don't embarrass the bride - if in doubt check she's ok with it
Make sure you're talking loudly enough to be heard by those at the back - ask the bride if you'll have a microphone so you can adapt to that - and slowly enough that slightly inebriated people will still be able to follow
Look up at the audience frequently, even if you need to return to your cards. If you feel shy about addressing strangers find out where your particular friends will be sitting and look around at them
End with a toast
Good luck, and have fun with it!