I'd done three summers of research (inorganic chem, neuroscience, then immunology--so you don't have to have continuity at all!), and I was working on the third project (full time) through the fall as my senior thesis. Somehow it'd never occurred to me to do research during the school year other than my thesis...
And to add no original advice whatsoever, I'd say knowing your work is more important than having done a ton. Also, though, it's not only knowing your research--being able to explain how you contributed/thought through some of the problems/figured out how to analyze some of the data yourself is probably key.