Figure out the tone of the roast. Determine if it will be a light-heated, family-friendly tribute or a raunchy, profanity-laced affair.
Pick the most humorous characteristics of the roastee. Think of his likes, dislikes, habits, strengths and weaknesses. Exaggerate these characteristics. For example, if someone is frugal, depict him as a greedy cheapskate.
Keep the crowd in mind when writing jokes. Some jokes will play better to people who also know what you know about the roastee.
Limit the roast to 4 to 6 minutes. It is relatively easy for people to laugh at a few short jabs at a person's idiosyncrasies. The longer it goes on, you run the risk of sounding like you are berating or degrading the roastee.
Use your own personality as an asset. Tell funny stories about the things the two of you have done. Reveal anecdotes that are humorous in retrospect. Tell comedic stories that reveal significant things about the honoree.