Watch the Alcohol Intake
The biggest factor that will send a well-intended toast south, and fast, is imbibing in too much bubbly, wine or beer. And heaven help you if there's a full open bar. Liquid courage helps you face a large audience, but it can also result in you becoming the butt of the joke. You need to know your limit and stay well under it until after you give your toast. If you have trouble managing these things at fun events then enlist a friend to help keep you in line.