try farmer walks, grab 2 heavy dumbells off the rack. Hold them by your side and take off for a walk around the gym. don't go to far at first because you have to get back to where you started before dropping them. It worked good for me and is kind of fun. it is good for shoulders too.
A little off topic, but while you work on strengthening your grip, you could use straps to help you get a couple more reps in. For example, if you find that your grip gives out on deadlifts or pullups before the muscles your are trying to work, use the straps on your last set and it should up you crank out a few more reps.