Well, unfortunately, that's exactly what you're going to have to do. You'll need to practice, practice, practice.
I'm kind of curious though, if you despise her fiancé, and she, he, and everyone else knows it, why are you the maid of honor? I can't imagine that you fully support their union. There's a big difference between supporting your friend even though you completely disagree with her choice of man, but going so far as to be the maid of honor for a wedding that you aren't even fully behind? That seems a bit odd. Hell, if I was him (the fiancé) I might have put the kibosh on your participation as such. I wouldn't have banned you from the wedding or anything, but the maid of honor and best man are supposed to be the one's fully supporting the marriage between the two. Not sure how that's possible if you despise the groom.
With that said, perhaps you could try a different route. Talk more about her and how much she means to you and how happy you are that she's getting married and that she's happy, and mention that you believe their marriage will last because she's found somebody she wanted to spend her life with. I'm not saying ignore him altogether, but make it a more personal speech about her and then wish THEM the best and offer your support at the end.
You're in the wedding, you have to step up and suck up your personal feelings. This is about to be her husband so you're stuck with them, not just her.
Tell her not to marry him! Its not too late.
what would that accomplish?
Or, if you wish to come out sincere, you could note that you've had your differences with him, but you wish them the best and believe she's made a great decision? ...You do hope them the best, don't you?
Panama is right, just say what's true. Word things carefully, sure, but there are some things that sound good in all wedding speeches that I'm sure you feel even if you do hate the groom. Say things like, "I hope you two are happy," or "I know you make each other happy." Things like that. As Nakki said, maybe mention past differences, but in the context of them being over and done with. If you don't want to approach the subject of the past at all, stick to hopes for the future. :)
I'd say just don't mention your past differences. The bride (although she knows) probably doesn't want to be reminded of them on the day of her wedding. I agree to say things like "wish you two all the best" and to talk about the bride/what a wonderful woman she is/etc.
It's not about you, you know. It's about them two.
If the marriage doesn't last, then you can go celebrate (though I'd think you'd rather be sad over something like that). But if the marriage DOES last, be happy for your friend. She found true love, and you should honor that.