The more tiles that pass through your hands the more chance you have of getting one of those really useful ones -S, blank, J, Q, X, Z. You will need to evaluate your rack; sometimes a particularly good combination of five or six letters is worth keeping, but generally, where you can, try to play four or more tiles on each move.

Evaluating your rack is a real skill that you pick up if you play regularly. Try to judge which consonants fit together. An obvious example might be to retain C and H, or S, H and R. Another example - N, L, and R are all good letters but if all are kept they need a hard consonant like a D, T or G to make them more usable.