i enjoy working out on a campus board. its a 45 deg. layback board with 6 horizontal holds just large enough to get the tips of your fingers on. you then hang from the bottom rung, and work your way up and down.
do this a couple times a week
oh, did i say enjoy? actully, its torture.
I also do pull ups on door frames with my fingers and dead hangs on door frames with my fingers.
put a couple pillows on the ground, then hang from the top of the door frames until your muscles FAIL. not until it hurts, but until they shut down and you fall onto the pillows.
the rope, bucket and weights idea works very very well.
the #1 item to remember when increasing grip strength, is that this is primarely a TENDON strengthening program, and it will take a LONG time. after 6 months i'm finnally starting to see improvements.
you also need to take it very easy, doing some of these excersises can greatly damage your tendons if you have not worked up to them.
start with the bucket, then move to dead hangs, then start doing some finger tip pull-ups, and when you can finnally do about 10 you can start going up and down the campus board ONCE a week.
when you can make it up and down the campus board 3 times without MAJOR pain, you can do it a couple times a week.