How long since your last pregnancy? My cycle changed quite a bit since my daughter was born -- I ovulate later and my luteal phase is shorter and not consistent (which it's supposed to be).
Anyway, if you can afford to spend the money on it, I highly recommend the clearblue fertility monitor. It takes the guesswork out of OPKs, you don't have to time things correctly (I've read a lot on the internet that you should test at like 10am and 2pm with the OPK and that wasn't possible for me, I work full time). With the fertility monitor, you just use urine from first thing in the morning and it will tell you when you have a high and peak chance of ovulation.