Check this out:
Wikipedia Entry for American Football Strategy
I just found that, and it's pretty good for strategy, but not perfect.
It really all depends on the type of team you have. I use the Ravens, so I'll tell you my strategy with them. I run a lot of runs up the middle (running plays with the word Slam and Dive, mainly, and sometimes Counter's too). I avoid stretch plays because Jamal Lewis isn't really made for those types of runs, although he can bust huge runs off of toss plays from time to time. I also like to run a lot of short passing plays where the receivers run cross/slant/curl/out routes with the occasional fade route. After you establish the run, run a PlayAction play and you should usually be able to find at least one open receiver, and if you're lucky it'll be one that's running a deep slant or fade.
On defense I run almost exclusively zone defense's, at least until 3rd & less than 6. I also like to confuse the offense by blitzing different combinations of linebacks, and I especially love blitzing with my Nickel-back.
Hope that helps