Keep a balanced rack
A rack with only vowels or only consonants is hard to play. Aim for a mix, usually three or four vowels among seven tiles. After a turn, think about the tiles you keep as much as the word you play. Frequent letters such as E, S, R, T, N and A combine with many others; a pile of duplicates or awkward letters does not.
If your rack really does not work, exchanging tiles can be better than playing a weak word. Most games allow it, though it costs your turn.
Learn the short words
Two- and three-letter words let you play tiles in tight spots on the board, and they let you make a second word at the same time. Many players who know the short words score better than players who know long ones. Learn the two-letter words of the game you play first, because they differ per language and per game.
Use hooks
A hook is a letter you add at the front or back of a word that is already on the board, such as an S to make a plural, or a D to make a past tense. The hook letter becomes the start or end of a new word that you play at a right angle. You score both words, and the letter that hooks often lands on a premium square.
Play next to words, not only across them
A parallel play places a word beside another word, so that several two-letter words form at once. It can score a lot with ordinary letters. It is also where the short word list matters most.
Plan around the premium squares
The coloured squares multiply letter or word scores. A modest word over a double or triple word square often beats a long word without any bonus. Also watch what you open for your opponent: a word that ends next to a triple word square gives them a chance.
Go for the bonus when it is there
Playing all seven tiles in one turn earns a bonus. In Scrabble it is 50 points; other games use another number, usually in the same range. Letters like E, R, S, T, A and I often combine into seven-letter words, so keep them when they come together. Putting your rack into the unscrambler with the option to use all letters shows whether a seven-letter word exists.
Use the tool for learning, not only for winning
Using a solver in a game against another person is usually against the rules of the game and of friendly play. A better use is afterwards: enter the rack from a game you just played and see which words you missed. Over time you start to see them.