1. Write it well in advance.
You are going to get busy, and you are not going to want to/have time to sit down and write proper vows. This is where I messed up. I kept thinking I would have some time to get it done but stuff really piles up that week. I just kept putting it off, and then finally did it about 45 minutes before the ceremony was about to start. Now I'm a solid writer, so they turned out well, but they could have been better. Do not make the same mistake I did.
2. Keep it short.
Mine was probably about a minute, but I hit the key points, made sure to take her cocky ass down a peg but was nice enough where she wasn't going to call the whole thing off.
3. Make it sincere.
Even if you are a shithead like me, you should still make it sincere. You're only going to do this once (fingers crossed), so you should do it right. A real key to these weddings is that this is not your special day.
4. You can add humor, but make it appropriate.
If you're not using humor, this should be easy for most people, but adding it in with sincerity is tough, because many don't talk like a lovey dovey smoochie smooch when with their lady. I am definitely one of these people. Honestly, most of my private conversations with her make us both look borderline retarded, but we fit well with each other. Like I said, I was able to take her down a peg, but it was still 90% complimentary.
5. Memorize it, but be prepared.
You should take the time to memorize your vows. It will look way more baller, and maybe people will think you're just super romantic and these words are coming straight from your heart. With that being said, be prepared for the possibility that you will get nervous and forget everything. Pack a notecard, and hopefully you won't have to use it. Since I'm an idiot and wrote mine 45 minutes beforehand, I definitely needed my notes, but I just used simple notes so I could still talk to her. Still, it is a nerve-racking day, so remember to go slow and actually think about the words you are saying.
As I said, this is one area where I definitely could have improved, but I hope you can learn from my mistakes. Oh, also, if you are looking for a great quote to use, my bride used, "Life's cutthroat, not love boat," and there is not a situation where that quote is not awesome.