Keep it short. Unless you're a master storyteller, holding up your glass for longer than a couple of minutes distracts a whole-hell-of-alot from the toast.
Be sincere. Find something positive to say even if you're not 100 percent sure this is a match destined for the ages.
Keep it light. One funny remark is great, more than that and it starts to sound like a standup routine.
Don't say anything creepy, weird or disgusting.
Make it clear when you're finishing up - pick up your glass, hold it aloft, and make sure everyone knows when you're making the formal toast.
Do not, and this is important, aim to be the bit of the day that everyone remembers. You are aiming for "best supporting actor"... the bride (and, to a lesser extent, the groom) are the people who need to keep the limelight.