How can you make a girl blush?
Coming from a girl, I love it when my boyfriend says "you look REALLY GOOD today!" (extra emphasis on the really an the good)!! I guess it's a way of telling her she looks hot but your not being "offensive"
Tell her nice sweet things
Complimenting her... But don't flatter... That's a sin
Wink at her ;) or bite your lips in my opinion