Go with Red stuff that hoses White, like Flashfires, and Blue stuff that hoses Green, like Hibernation. Also pack some anti-Artifact hate like Shatter. Outside of that, without knowing anything about your deck's contents beyond the colors, it's hard to say. You didn't even mention the format, or your local metagame.
Most sideboard decisions revolve around shoring up your decks weaknesses against certain matchups and the prevalence of those matchups. Every time you find yourself saying "Damn, I wish I had something in here to handle that!" or "Groan...Another autoloss!" you've found another sideboard need. Make your sideboard decisions based on that, and you should be OK.