I have never given one of these (though maybe that will change soon — congrats @Mary and @bart!!!) but I have listened to my fair share.
  1. Don't make a speech about how you are closer to the bride (or groom) than anyone else
    Classic maid of honor mistake. You got picked over all the other bridesmaids; no need to rub it in. (ht @kirit)
  2. The best thing you can do is make everyone who doesn't know the bride/groom feel like they do after you gave your toast about them
    Half of the other rules on this list are to point you in this direction.
  3. Don't tell the story of you stressing about and then eventually writing the speech you are giving.
    Time may be a flat circle but wedding toasts shouldn't be! Related: I don't need you to tell me the dictionary definition of "marriage" or any other word in this context.
  4. Show don't Tell
    Your 4th grade teacher told you this when you wrote your first take home essay. It's never been more true than in this moment. TELL ME A STORY DON'T RATTLE OFF A LIST OF ADJECTIVES. Even if he/she is the kindest, most gentle, smartest, most determined, funniest, with the best sense of humor, most gorgeous, and a rock in your life.
  5. Go easy on the inside jokes
    The point of a wedding is to pull everyone in together and make them one group around the couple. Inside jokes are the best jokes but not in this setting.
  6. Things that are OK: saying "I'll be brief". Promising not to cry. Or announcing that you promised yourself you wouldn't cry. And then obviously crying.
    Not advising any of these but I wouldn't hold any of them against you.
  7. Tell a story (or 3) about the bride/groom
    Since you're at the wedding, there is a built in happy ending! No spoken words are as compelling to human ears as narrative.
  8. Don't mention exes of either party
    Even if it's really funny and everyone is on good terms. Just not the right time to have anyone thinking about those people.
  9. Don't be too intense or too embarrassing
    Everyone wants to dance to Michael Jackson on the parquet after this. Telling about how your son wanted to commit suicide when he was 16 or making your daughter so embarrassed she runs away and hides are going too far, even if you're speaking the truth! Keep it lighthearted except around the emotion of love.
  10. Run your speech by somebody who will be very honest w you before giving it
    You might think something is super hilarious and will go over really well but you could also be very wrong and that would be bad.