A good starting point to decide where to play, is to ask yourself; what is the highest score I can get? However, it's not the only consideration. You need to think whether you are getting value for the tiles you play and consider what letters you are leaving on your rack.

The best tile is a blank. It vastly increases your chance of getting a 7 letter word and collecting a 50 point bonus. As a rule of thumb a blank should be held unless it increases your best play by upwards of 25 points. Think about setting target scores for the other letters such as S, J, Q, X, and Z.

Try to keep a balanced rack i.e. a rack that has similar number of vowels and consonants. Try to play duplicate letters (if you have 3 N's you may take a few points less in score in order to be able to play one of them.) Try to keep the letters from the word RETAINS on your rack as these are the letters most conducive to making 7 letter words and getting those elusive 50 point bonuses.