Triple word squares are crucial. So don't make them easily available to your opponent.

However don't be so concerned about these squares that you ignore good plays that open up the triples. Try to weigh up how many you score against what your opponent is likely to get. A play which covers the double letter square which is to be found 4 or 5 spaces from each triple will considerably depress the scores available on a triple and make the likely score from your opponent much lower.