If you're too broke to purchase the Captains of Crush (COC) grippers---or something of the sort---I can save you money and strengthen your grip. As well, if you own the (Captains of Crush) grippers and are having a difficult time closing whichever one, this method will help as you can gradually increase the load.
You'll need a cable apparatus where you'd normally do cable flyes and such.
1) On one end, attach a handle as you would, as if you were going to do flyes.
2) Remove any handles or bars from the other side so only the hook/clip attached to the cable remains.
3) Pull the cables together, placing your thumb in the hook/clip, and your other fingers on the handle.
4) Have at it.
Obviously, you'll need more weight for the fingers and less for the thumb. I've found that it's easier to pull with the fingers first, doing the thumb separately. If you attempt to do both together, your fingers quite likely will be doing the majority of the work.