I don't do anything too specific for grip training, but I do like to challenge it when working other things such as pull ups. I also have a rope that I try to use as much as possible for pull ups.
I use a rice bucket when I'm training handstands, and I also work my extensors with a thick rubber band. It looks funny when I do that on the train, I bet women get a little scared thinking I'm a professional boob groper or something....
My grip is still somewhat shit though, but nothing comes easy.
I used to seriously post here, now I prefer to troll.