A lot of my friends who have game talk about playing hard to get and keeping the mystery alive with a girl until they've snagged her. They say things like "let her come to you", but in my experience most girls don't make the first move or second or third. How do you know when it's time to be assertive and when it's time to hold back and wait?

I'm thinking expecially of situations where you've already approached or know the girl, but haven't had a date yet.