Choose your blue flag posts carefully. Not every post is a good blue flag post - particularly if it's coming into a corner. Other than that, practice.
I agree you shouldn't overuse blues - but they are only an advisory flag - you'll never get good at it if you're so afraid to put out a wrong one that you never put one out at all (I don't necessarily apply that to a race situation, but certainly in qualifying). The drivers will forgive you the odd wrong one as long as you seem to be getting better
