There's a highly effectivy method called symptothermal method. It is basically a combination of temperature measuring and cervical mucus observation, as both symptoms change during the menstrual cycle and when ovulation occurs, you can tell from the symptoms. After you successfully determined that the ovulation passed, a woman is infertile until her next cycle starts.

This means that during that time, you don't need any condoms or anything.

Unfortunately, while being extremely well-researched in Germany for more than 30 years, it is quite uncommon in the US and only known as "Fertility Awareness", which is not the "pure" symptothermal method. The method is oftentimes associated (wrongly) with Christian groups.

You can check out Wikipedia:

http://en.wikipedia.org/wiki/Fertility_awareness

Edit: Perfect use application of the "pure" symptothermal method has a Pearl Index of 0.4 (99.6% effectiveness) and typical use is 1-3 (97-99%). This is quite comparable to the pill.