I add just a smidgen of rice cereal. Our pedi had us adding a large amount when he was younger because he had reflux. He's since outgrown the reflux, but I found that adding about 1/2 a tsp to a bottle keeps the formula from clumping - even if you use cold water like we do. Water first, then cereal, then formula... shake like crazy.

I wouldn't do this with a really young baby though unless advised by a pedi.