Here are some things to keep in mind when toasting the new couple:
1. It's a toast, not a roast.
Of course you have embarrassing stories about the bride or groom. If you didn't you wouldn't have been selected to speak before 150 of their closest family and friends. That said, a wedding is not the time to tell everyone about "That one time when..."
You can use humor, but make sure you use it tactfully.
2. Keep is short.
If only three people give speeches and each runs approximately three minutes, that equates to almost ten minutes of talking (including getting the guests to settle down enough to listen and passing around the microphone). Unless you've cleared it with the bride and groom ahead of time, try to keep speeches to three minutes or less so everyone can get back to partying.
3. Keep it simple.
This is not the time to prove how well you know the bride/groom/couple by diving into a detailed history of your relationship. Instead, focus on one or two points you want to make about the bride/groom/couple and then offer your best wishes for the future.
4. It's not about you.
Yes, you want to present yourself in a charming and charismatic way, but remember it's not your day. Focus your toast on bride/groom/couple and make sure they're the hero(es).
5. It's okay to use notes.
Speaking in front of a large group can be nerve-wracking for many people. If you're worried you might go blank when standing there with the microphone definitely bring notes with you - or even a printed copy of your toast. Plus your note cards or printed speech make a nice keepsake for the new couple.