Every wedding has them -- toasts, where members of the wedding party (and sometimes others too) say a few words (if you're lucky) about the bride and groom. There are several things to keep in mind, though, when it is your turn to make a toast.
1. The number of toasts. Toasts usually come right before, or during, the time when people are going to eat. Your guests are hungry, most likely, so keep the amount of toasters to a minimum so they can get to the food you've certainly paid a lot of money to have served. (It's also usually the case that the caterers have the food ready to go and holding them up with a lot of toasts will cause the food to start to get cold). I recommend no more than three toasts (maid of honor, best man, and perhaps the father of the bride or one other). Any more than that will get the rest of the guests bored, and often the content of the toasts becomes unoriginal after that point. Some strategies to incorporate more speakers is to have some give a toast at the rehearsal dinner instead, or give a toast as a group (i.e. all the bridesmaids, all the groomsmen, each say a quick word).
2. Length of each toast. Just like the number of toasts that occur, you don't want two really long toasts either for the same reasons, but also because often times, the content of the toast are a lot of inside jokes and stories that a large percentage of the guests will not be able to understand. With this in mind, keep the length to no more than a minute or two at most.
3. Plan something to say. No one is ever told on the day of the wedding, "You're giving a toast." There are plenty of websites and other ways to get inspiration for what to say in your toast. Even if you're an off-the-cuff kind of person, and you think it's more "honest" to have it come straight from the heart that day....don't. It never ends up sounding good, trust me. You don't need to memorize the toast either. Put it on your cell phone, or read it off paper, but if you want to look more spontaneous, just remember the main points you planned already, and go from there.
4. It's a toast, not a roast. It's a happy occasion with some of the bride's and groom's closest family and friends there, so keep the content light. Don't relate a story which could potentially embarrass someone, or worse, might shock a guest, and not in a good way. For some reason, many toasts evolve into this. While some good-natured ribbing is always fun, keep it focused on the bride/groom's merits. If you'd like to get specific, limit it to one quick anecdote. No one needs a history of the bride or groom at this time.
5. Microphone etiquette. I've posted about this before on here, but when you toast you need to hold the microphone correctly. Otherwise, what's the point of giving a toast if you can't be heard? Microphones still need to have a loud voice to amplify. In general, your voice should be just above your normal speaking volume, and the mic should be a fist's distance from your mouth.