Eat like no one's watching.
Go ahead and order the 2-for-1 couple's special at your local pizza joint. Eat that pint of ice cream straight out of the carton. Treat yourself to an entire cheesecake.
This is your day, and you make the rules.
After all, in case you didn't know, when you're single, all calories consumed on Valentine's Day don't count, so indulge!