We've all had times in our lives where we did things to embarrass ourselves, right? Generally, retelling these stories is likely to add humor to almost any conversation or situation. However, your maid of honor speech is not necessarily the time to bring up such stories. Telling about when the bride wet her pants at the school dance in eighth grade isn't the best idea -- especially if she's sworn you to secrecy. A little embarrassment is funny for the group, but be careful not to go too far with your jokes. You don't want to hear crickets when you should be hearing laughter.
You should also avoid stories involving drunk, high or deviant behavior. It's definitely not appropriate for the setting. The goal of your speech is to celebrate a friendship and a new marriage, not to air your friend's dirty laundry.
Here's a good rule of thumb: If you can't talk about it in front of your mom, you probably shouldn't talk about it at all.