Liquid or spray stuff from autopartstore. Supposed to make old dry cracked belts OK again, but it's really only a temporary fix.

But now you say the belts are relatively new. New belts stretch a little the first week or so. Maybe they just need to be tightened? The reason they only squeal on startup is...
1 - They're cold.
2 - When starting the engine that's the one time the belt is suddenly started up. Puts a little more load on the belt vs. driving or revving the engine.