A woman can get pregnant in any position. She can also get pregnant even when semen leaks out of her vagina, as about 150 million sperm are deposited on ejaculation, and only one is needed to fertilize the egg. However, lying down horizontally for 15 minutes or so after sex won't hurt anything.

It is thought that positions like missionary or from behind aid in conceiving quicker because of depth of penetration possible in these positions, allowing sperm to be deposited deeper against the woman's cervix.

Most importantly when trying to conceive is proper timing. Try having sex every other day all cycle if you aren't sure when she ovulates, or every other day from cycle day 8 up until cycle day 16 (if she has regular 28 day cycles with ovulation occurring on day 14).