You should be using condoms at the very least to prevent pregnancy. Using plan b as contraception is a very bad idea because it isn't meant to be used that way. It's meant for emergencies, not for casual use. Girls who use it all the time mess up their hormones and usually end up pregnant anyway because they took it wrong.
If your trying to prevent it use protection or don't have sex the pill is for accidents only not if you just are careless and don't use protection for three days in a row