For awhile I have been using a Gripmaster, like one of these:
I keep it on my desk at work and frequently grab it when somebody comes in to my office to talk, even taking it to meetings on occasion. And yes, it does intimidate some people that I do this. Frankly, I don't care. I forgot where I read it, but these things are recommended by FBI firearms instructors.
Anyway, after my most recent and rigorous dry fire exercise, I decided to do dig a little deeper on this subject, and low and behold... I'm doing it wrong.
According to this link, there are three types of grip strength: crushing, supporting, and pinching. The grip master is really great for crushing and pinching, but supporting grip strength is more vital for pistol shooting and self-defense. Here is a good article on general grip strength and here is a video on how to properly use those V-shaped hand strengtheners (yes, there is a right and wrong way to use them). Note that the first article recommends using barbells with 2.5 inch diameter bars instead of the typical 1 inch bars because "it simulates grabbing a man's wrist" (I'll note here that most handguns have grip areas larger than 1 inch diameter as well).
For exercises for pistol shooters, you need to read this column from Pistol Australia (yes, that's right... gun-law happy AUSTRALIA). It points to studies in this area, but the larger point I see in it is that proper grip strength is related to strength in the entire arm. Notice how the exercises include tricep extensions and curls in addition to wrist and forearm training. Interesting stuff.
Looks like there are things I'll be adding to my dry fire training.