In reply to Exile:
What I've been doing is a comblination of the above - dropping off and getting straight back on, but then leaving it 5 - 10 minutes ish between sets.
If I leave shorter between sets won't I be in effect training endurance? (I'm not sure I could leave less time on the difficulty of problems I'm doing as I'm pretty shot after each set, and rep three and four are often touch and go.)