This is not too hard to do as long as you do not fail a date with a particular girl, remember to date her every day and time the 4th date correctly so that it takes place at night (apparently girls can only have sex at night, otherwise they turn into hedgehogs). Do not put all your chips onto just one girl, try to have at least two ready to roll in the hay on day 4, because the cheerleader often sleeps early and you might miss the window on the 4th night. Remember to upgrade your stats, especially the attribute(s) dearest to the girl or girls you are trying to bed. Also, passion is always a good investment.