I usually try to use a 50:50 (or 75:25) mix of mozzarella to provolone. Sometimes I'll even add a very small amount of cheddar.
I was at the store the other day and Kraft now sells bags of shredded "Pizza cheese" which is a blend of mozzarella and provolone.
Speaking of Kraft cheese... the latest issue of CI magazine just performed a blind taste-test of mozzarella cheeses. It's a very good article and an eye opener. They tested most of the major brands (block and shredded, whole milk vs. part skim, fresh vs. mass produced, etc., etc. Tasting was done with raw cheese and cooked on a pizza. The best was Dragone whole milk mozzarella (not available everywhere), but coming in a very
close second place was Kraft shredded low-moisture part-skim mozzarella!
The results surprised them as well! Surprised me too!