The best ones I've witnessed have all been short, funny and honest. The worse was when the bloke made a huge effort to try to be memorable, but really I don't think these speeches are meant to be. If you do decide to write the whole thing out, definitely read it out loud lots of times before the day. Not just to help you remember it, but because it'll help you to find any awkward, hard-to-say parts that might need rephrasing a bit until they trip off the tongue.