Radix capture it right. There just no real training regimen besides getting your stamina as high as you can and some strength/endurance exercises. Climbing ropes, strict deadhang pullups (that means no kicking/swaying whatsoever), doing calisthenics/aerobic exercise till your eyes turn blue (jumping jacks, leg lifts, pushups, body builders, & mountain climbers are key)
Yea the crunches is the easiest part of the PFT.. 3 miles less than 18minutes though is truckin'. If you haven't already started, start alternating your runs with sprints and distance running to get your time down. Another part about running they never tell you is that running in formation is cool - running in formation at a good clip is challenging - running in formation at a good clip while screaming a chant to motivate is whole another world.. practice singing those songs while you work on your running. It takes rhythm to keep time and inspire folks around you yell with you.
And not to dis the Army or anything but humpin with the suck is wild animal all to its own. Forced marches can go as high as 25miles and the 3mile to 4mile an hour pace feels like a mad-sprint with 70-80lbs on your back. That's not the bad part - its the blisters/callouses that are the backbreaker. If your feet are not built 'Ford-tough', start practicing and getting the pain of blisters out now in the comfort of your home while you can. Moleskin is a life-saver as well.
The physical part of OCS/Bootcamp isn't mindblowing tough - it is the mental games, constant repetition, and the competition with your peers that you have to fight through. It's one thing just completing - its another thing to excel. No gym routine is going to make you 'want it' and that drive is what will make everything else go smooth.
Good luck to you and Semper Fi