as low as possible as high as necessary. But that's secondary.

The key really is to think ahead. While you're playing a note you're already placing your finger above the next one. Another tip is to relax. The more you tense up, the more your fingers will freeze and tend to stiffen.

another good exercise which i picked up from burbridge is like the chromatic 4 note per string thing except instead of chromatic he does whole tones. This is to practice hand movement instead of training finger strength and stretching. Moving left hand positions is also something you need to practice and not just finger speed