To do a correct Marine Corps crunch, lay on your back with your feet flat on the deck, and your butt close to your heels.  Your arms should be crossed on your chest or your stomach.  They cannot come off of your chest/stomach during the PFT. To do a correct crunch, raise your upper body off of the deck until your forearms touch your thighs.  Go back down until your shoulder blades touch the deck.  That is one crunch.  To max the crunches on the PFT, you must do 100 in 2 minutes.  There are several different ways to train for the crunches.
Endurance Sets
To be able to do 100 crunches in two minutes, you must first develop the endurance to do 100 crunches.  Get into position, and do 100 crunches without keeping track of time.  Focus on proper form.  After you do 100, rest for a couple of minutes and repeat.  You should do 2 more maximum effort sets, building up to 3 sets of 100 crunches.
Speed Sets
Get into position, and do 30 crunches as fast as you can, while still maintaining proper form. Rest for 1 minute and repeat.  You should do 5 or 6 sets in this manner.
Incline Sets
If you have any access to an incline board, get into position on it and do as many crunches as you can, until you cannot physically complete a complete crunch.  Rest 1 minute and repeat.  Do 3 to 4 sets in this manner.