PFTs should be routine, especially for someone in a tactical unit. There should be no particular prep needed. My warmup every training day now is 50 squats, 50 push-ups, 50 sit-ups and 10 chins. I did the same when I was still serving although the reps were higher. This is your warmup and then you move on to real work.
Build up to the number you want, I figured 80% of what you wanted on the test is a good number. Start wherever, 20 reps? Something easy. After two weeks or so add five reps. For chins or pull-ups I always use 1/5 of the push-up, sit-up number. Obviously if your max is five chins you're going to need to scale this.
Keep it easy, add reps until you get to your target number and do the warmup every training day. 80%+ can be, should be, easy.