if birthcontroll is out for you and you are taking a chance on having unpertected sex then I suggest you track your cycles and your ovulation such as a woman trying to get pregnant, this way you can avoid those days most fertile. this website has a great ovulation tracker.
I agree with Annie - for this method to be effective, you really need to monitor your menstrual cycle closely and know the signs your body gives you that you're about to ovulate. Otherwise, it is less than 60% effective. And if you happen to have irregular cycles, it is even more difficult to determine when your most fertile time is.
The theory is that at mid-cycle you are most fertile, and near your period or on your period you are less fertile. However, there is a lot of variation -- real women just don't fit the average.
For example, a woman with a 28-day cycle from the first day of her period to the first day of her next period:
Day 1 of period, for example, January 1
Ovulation would come, for example, January 14
Day 1 of next period, for example, January 28 or 29
As I said, so many women do not fit this kind of calendar that it is almost futile to write it out. But the theory is that two weeks before your period begins, you ovulated.
When you ovulate, you have about 24 hours of fertility. Unlike men, women have a relatively small time window in a month for a viable egg.
Men's sperm lives 4 to 5 days in the woman's reproductive tract. In the example above, a couple having sex on the 9th or 10th of January would still not be protected from pregnancy even if the woman didn't ovulate until the 14th.