I've recently returned to playing Letterpress & I am off to a good start winning nine games on the trot. I've taken a lot of advice from Jochta's recent post on UGVM.
Always put an S on a word if there is one.
Variants like _ed and _er are counted as different words.
Look for endings like _ing.
Once you have control of the important letters and a corner build out into the board and take off unused letters one or two at a time to limit your opponent.
Leave Q to the end if there's no U as you know your opponents can only play QI or QIS with it.
Try and play an anagram (or nearly) of your opponents previous word.
Never leave common letters uncoloured with a close score as your opponent will almost certainly be able to polish them off.