I was looking for methods to increase my grip strength and just bought these GiZilla Grappling Grips online about a month ago to try them out. I know you can use a towel for a lot of the workouts, but my wife gets ticked off when I take her towels and ropes are terrible. These grips are really strong, made out of Heavy Gi Material and Seatbelt like material and I like that they work with all of the machines at the gym. If you are tired of rope burn, hearing it from the wife and still want to incorporate grip strength training into your workout, I would check out these GiZilla Grips.
Towel pull ups are the tits, but you can also used you belt or gi wrapped over the bar. Ropes, farmers walks (even better with fat grip dumb bells), captains of crush grippers. Tire flips, but use just your finger tips half way up the tread.
Sandbag training and not the commercial ones with handles. Bucket of rice, plate gripping and rope climbing.