This is something I learned when I did my CrossFit experiment. I was strong, but whenever I had to do a WOD where there was lots of pulling or pull-ups, my grip would just give out.
Why would I fail after 10 pull-ups while someone weaker than me could do 30 kipping pull-ups? It's not hard to figure out. To do 30 pull-ups (kipping or otherwise), you have to hold on to the bar for at least 40 seconds, possibly as long as 60. But when I tried to just hang from the bar, I could barely hold my bodyweight for 20 seconds!
That indicated an embarrassingly weak grip, so I added 2 minutes of chin-up holds at the end of my workouts. Here's how it looks:
Chin-Up Bar Holds
Surprisingly, it doesn't create fatigue. It might even help recovery by decompressing the spine.
At first it took me 8 sets or so to add up to 2 minutes, but eventually I was able to do it in one long set and one small one. It's a very simple way of increasing grip strength.