The Finger Curls never worked for me personally... You just have to keep climbing!
Find a climbing buddy that is committed to training with you at the climbing gym. After you've completed your normal climbs/ problems and you're feeling pretty pumeped- go find a route that is hard for you but not too hard... Maybe two grades easier. Climb it on top rop- taking great care to have exceptional technique- and then down-climb it all the way down until you reach the bottom-most foot holds. Then repeat the route about 4-5 times (climbing up and down). Then do the same for your climbing buddy. Then go find another route that has a different flaver and repeat. And then again.
Go for the longer routes and avoid over-hanging ones until you are stronger. Avoid slabby routes- as they do little for your grip strength. (You can work on your technique earlier in the day.) Look for routes with lots of slopers. Slopers are the key for building a strong grip- even for improving your grip on tiny crimpers. It sounds crazy but I swear it is true.
If you don't have a good partner- do a lot of traversing in the gym. You obvioulsy have to push yourself even as your forearms are swollen with pain. Wrapping up your climbing sessions by hanging on a hang-board is another tid-bit that helps build the strenght. (Include pull ups if you can.)
Finally, it sounds lame- but traversing back and fourth on playground monkey bars is also a good climbing excersize. You can also build your own overhanging board in your home/basement to do hand traverses.
I always knew I had a good day when I could barely grip my steering wheel for the drive back home after the gym. That being said, you will want to buy velcro climbing shoes rather than lace ups. If you are training hard- laces will be impossible to untie for several minutes.
Claude- awesome photos, by the way! The skinny crack looks HARD! And speaking endurance: It appears that the climber in the bottom picture has some serious grip- endurance as they kept going past the anchors. Good Stuff!