Because this question seems to come up from time to time, I just wanted to clarify:
You can enter each contest on the site (the open ones, anyway) once a day.
You don’t have to choose just one contest to enter each day: if we have twenty contests open today (I’m too lazy to count the real number right now), you can enter all twenty today, tomorrow and until they close. Don’t stop with one, hit them all each and every day, then come back tomorrow and hit them again. Make it part of this balanced breakfast.
Hope that helps you with your contest-entering frenzy. Go nuts.