second the towel suggestion, you can also double the towel over a bar and grip the two ends together to make an old school grip trainer. challenge: break the towel, next time wrap it round the bar an extra time (new towel obviously), break it again…ad infinitum
also second the above that fat gripz are worth the money, solid as a rock, you can use them for pushing movements to take some pressure off the elbows, or put them in the middle of the bar to make a low board for bench. i like to use mine on warmup sets with almost all pulling movements (i never dead with them, but cleans are crazy), whip them off and you will rip those work sets with renewed confidence, unless of course its the first twenty times of using them, in which case you will clutch your forearms with a look of constipated anguish on your face until its time to do the next set…or at least i did.
also just get a heavy sledgehammer and swing that beast around. i got a 14lber a couple months back that is murdering my grip right now. aside from the inevitable general sledgehammer fun i do windmills with it as part of my warmup, you definitely have to focus on gripping very very hard. with the incentive being that if you dont its game over for your feet or the neighbours fence…be careful! the only mishap so far was fortunately a glancing blow which only left two of my toes blackened but not broken.