First, you have to understand that in order to train
the grip, you not only do gripping style exercises, but you must also
train the muscles on the opposite side of the joint. Most of the the
exercises you probably do involve holding on to weights - this works
the flexor (gripping) muscles. Often athletes neglect extensor training
completely, so integrate 2 exercises to your grip strength program:

1) Reverse ez-bar curls

2) Rubber band finger extensions

For #2, you can use a thick
elastic band (the kind that usually comes around broccoli) or buy a
tool called the PowerWeb. Just stick your fingers in the band and try
to straighten and spread your fingers apart.

Now to train the gripping muscles themselves, you want to hit the 3 components: strength, endurance, and power.

Strength
of the grip muscles is generally not an issue if you're performing
exercises like heavy Deadlifts or Chinups. If you regularly deadlift,
you can probably lift at least 300 pounds - if not, get lifting!

Endurance
is another component that you need, so to build endurance, you can do
something I call Grip Finishers - simply do your normal exercise, let's
say a set of 10 chinups, then hold on to the bar with a strong grip for
as long as you can. This is best done on your last set, because you'll
ruin your subsequent sets by fatiguing your grip. Deadlifts, Barbell
curls, and Dumbbell lunges are also good candidates.

Now gripping
power is often not trained and can really help you out in MMA or BJJ
competitions. Power is force x velocity, so you need to be able to
apply your full grip strength quickly to develop a powerful grip.

My
favourite method of training grip power with my mixed-martial artists
is to do an exercise I call Plyo chinups. Do a chinup, then when your
chin is over the bar, let go of the bar and then grab on again as you
straighten your arms, then pull yourself up as quickly as you can. The
farther you let yourself drop, the better. But start off with a smaller
drop and work your way up - these are really demanding.

You can also use this method with 1-arm dumbbell rows.

Other
ideas to develop your grip are to do exercises holding onto towels
instead of holding the bar. This is great for chinups, dumbell lunges,
etc. If you train or compete in BJJ with the Gi then you'll definitely
benefit from this exercise.

You can also use a fat bar handle
instead of a regular dumbbell or barbell. Just wrap an old t-shirt
around the handle to increase its diameter and it will give you a
totally new challenge.

Whatever methods you do, make sure you
only do 2 weeks of grip training, then take 2 weeks off. Because you're
always working your gripping muscles, constant specialized grip
training can lead to overtraining or worse - injury, so be sure to back
off after 2 weeks.

Give those concepts a shot and you'll have a crushing grip in no time.