Know your ovulation cycle
Most medics would agree that women are most likely to get pregnant within a day or so of ovulating. Ovulation is when your ovaries release an egg ready to be fertilised.
This is usually 2 weeks (14 days) after the first day of your period, so having lots of intercourse at this time of the month is certainly doctors orders.
Whilst sperm lives for up to 7 days, eggs have a much shorter lifespan - they only 'live' for 12-24 hours once they have been released so you might want to schedule 'special' baby making appointments with your partner when you know you're at your most fertile.
Of course that doesn't mean you shouldn't go for it at other times during your cycle - don't worry about 'wasting' sperm - your man can regenerate a supply of sperm surprisingly quickly.
And as sperm live for longer, they can travel up the fallopian tubes and wait for an egg to be released - that's why it's so important to have regular sexy time.
Check out sofeminine's Ovulation Calendar Tool