It's stressful to have to do these, but it could be worse. I was once called on to give such a speech impromptu when the best man got lost for over an hour between the wedding site and the reception -- now there is some stress. I mostly made fun of the groom for being bald and paunchy compared to his hot wife. Everyone but him loved it, and I owed him one for putting me on the spot.
I actually practiced and timed mine. Came off natural though, a couple "wow, no paper, off the top of the head huh?" even. Kept tweaking and shortening it to stay at about five minutes. Like was earlier said, thank everyone for coming, don't forget that this is 1/3 let the crowd know how great your bro is, 1/3 how excited (or just fake this) you are to have the bride coming into your family and 1/3 say funny $#it.
My method has been to come up with humiliating stories about the groom, but turn it into "and that's why he's so great and you will have fun with him, (bride)..."
Take BMD's advice make it short and painless and keep it clean. Remember there are usually children, parents and grandparents in attendance. Doesn't have to be G rated but keep it at least PG-13. I've been to plenty of weddings and stories about the groom getting arrested and the sexual jokes don't go over too well.