Not all women's cycles are the same so when you ovulate may not be anywhere near when another woman ovulates. I have heard a week after your period, I have also heard (more commonly) 14 days after your period is when you are most fertile, but those are just statistics. It is going to depend on your cycle.
So, like the above post said to be sure and find out when you ovulate, buy a ovulation test kit.
You don't have to lie on your back for 15 minutes after sex to get pregnant, otherwise, all unwanted pregnancies could be avoided by standing up afterwards. But, it will definately help. Try not to pee right after sex, and stay laying down for a while. Also, wait a little while to take a bath also, just at least an hour.
And relax! The best thing you can do is relax! Just enjoy the sex you are having with your partner and don't stress about trying to get pregnant. Most people get pregnant when they stop trying, or aren't trying. So have fun!