Watch the Alt. Picking: 1 online guitar lesson by Dave Celentano from Classical Concepts for Rock Guitar
The higher section of "Prelude in C Minor" is a great study in alternate picking. Specifically, strict alternate picking beginning with an up stroke. This uses outside picking which is where the pick pivots on the outside of a pair of strings. Use the first two measures to rehearse this until you are comfortable with alternate picking beginning with an up stroke.