Very few people are natural toast-makers.  That's fine; it's not an easy thing to do. Being self-aware can help, though:

No one cares who YOU are.
I mention this all the time, because it is the most common problem.  Many, many toasts are about the Best Man/Maid of Honor's relationship to the Groom/Bride, instead of being a tribute to the newlyweds.  And so you'll hear things like:
"For those of you that don't know me..."
"We shared a lot of laughs in High School!"

You are not funny.
Really, it's ok.  Most of us (including me, President Bush, and Jerry Seinfeld) aren't either.  Just don't get caught up in trying to be.
"Is this thing on?" or, "Can you hear me NOW?"
"Marriage is a three-ring circus: the engagement ring, the wedding ring, and suffer-ring!"
"One good thing about marrying a lawyer: if it doesn't work out, the divorce is easy!"
The wedding ceremony was beautiful. Everyone was in tears, including the cake.^*

You don't know how to improvise.
Since most people don't practice speaking in front of large groups extemporaneously, it's probably best to prepare something.  The spirit is NOT going to move you, and alcohol only "helps" in that it gives you something to blame your incoherence on later.  Finally, don't bring attention to your difficulty by pointing out how good a previous toast was, as in:
"Jeez, that's a tough act to follow!"
"How about another hand for [Name of previous toaster]!"
"Well [previous toaster] really stole my thunder, so I'm not sure what I can add!"

You are not a poet. DO NOT WRITE A POEM.
Or, for that matter, a song.

Finally, a reminder:  everyone will be waiting for you to tell them to drink.  "To the Bride and Groom", "Cheers!", "Le'chaim!", "Salud!" or any of the others here will do the trick. More than once we've seen people still standing at the end of the night, frozen forever with their glasses full of flat champagne, absently mumbling "hear, hear!"
I'm going to leave this posting open-ended, and add to it as we go.  I also promise not to compare myself to Jerry Seinfeld again.