In reply to ed34:
Tips:
1. Read/watch as much as you can re reps; the 7/3 sort described above is great for endurance, but max hangs are good for increasing basic finger strength (find e.g. 4 specific edge/grip combinations which you can each just hold for ca. 10s, then do ca. 7 sec hang/2 min rest for all 4; try 3 cycles of this with 5 min between cycles).
2. Be prepared to have to adjust your workout in the beginning, and don't force it!
3. Warm up thoroughly before each session (not only fingers; the shoulders are really important)
4. You have to give your tendons and ligaments time to catch up with the rapid increase in muscle strength, otherwise you'll get injured. I'd start with every 4-5 days, and do an easy week every 4-6 weeks. You should be thinking in terms of months and years for best results, even though the improvement in the first few weeks can be massive.
Enjoy!