Balance creates choices
A useful rack normally needs both vowels and consonants. Too many of either sharply reduces the number of words you can form, even when the individual tiles appear easy to use.
Value flexible letters
Letters such as E, R, S, T, A, I, N, and L combine with many partners. Preserve them when a similar-scoring move would otherwise leave duplicates or an awkward high-value tile.
Judge the whole turn
Compare the points scored now, the quality of the remaining rack, and the board opening you create. The highest immediate score is not automatically the strongest strategic move.