Also, it's important that you take your temp at the same time every morning after at least 3 hours of undisturbed sleep.
Note that I used charting to conceive... I haven't attempted using it to avoid pregnancy yet. I started temping after my first natural period after stopping hormonal birth control though. I was able to verify ovulation in the first cycle. I think people have mixed results though - especially w/depo; I was on the pill.