Most people's first instinct would be to run for the nearest dictionary and frantically start memorizing words. Expert players tell us that doesn't usually help that much. Most scrabble games are won (or lost) based on how well you know and play a small set of crucial words. Or execute basic moves like spotting plurals and prefixes. Better choices include studying lists of two or three letter words, which can easily be dropped into a vacant spot on the board. Better yet, pay close attention to short words that use rare (and high scoring) letters, which are great for setting up a knock-down scoring play. Imagine dropping a high scoring tile on a triple bonus square... or a scrabble word bonus square.
While the seven tile triple-score "smack down" opportunities sound cool, good scrabble strategy involves ensuring you can regularly play a word every turn. In the long run, players that are able to fit a 3 - 5 letter word on the board every turn as the scrabble board fills up will clobber the dictionary masters. This is a mix of learning to see what the game gives you (even if it's a humble pluralization or dropping a prefix) and managing your mix of letters.
Most English words consist of a couple of consonants placed around a vowel; so while you need a couple of vowels to build words, a rack full of them will quickly become a real problem. I've found that odd consonants tend to be less of a problem than vowels - you can often find a little word (and often a high scoring one, as little words go) to dump extra consonants off on. A rack full of vowels, however, can seriously choke your play (since you need to depend on the board to provide any consonants.) Try to keep only about 3 vowels on your scrabble rack.
If your scrabble rack stinks, don't be afraid to lose a turn and swap the whole thing out. Or at least as much of it as is required to restore a workable balance. It's better to lose a turn swapping out the scrabble tiles than limp along for several turns with a poor mix of letters.
The best words to play are often already on the board; expert scrabble players know how to give them a little "tune-up" to get additional points. The letter 'S' is worth it's weight in gold here, since it is an instant plural for most of the nouns in the English language. It is often a legitimate (game approved) scrabble helper trick in most of the computer version of the game as well, since they accept S as a plural (regardless of what your English teacher said).
Building on the idea of "tuning up" words which are already on the board, it's worth spending a little time memorizing the common prefixes and suffixes. Most common words have multiple versions that can be formed with by adding 2 - 3 letters to the front or back. For example, consider the word MADE. It is contained in about 23 other words - including: unmade, premade, mismade, manmade, homemade, handmade, and readymade (wow! 5 additional letters down!). And be alert to opportunities to play both sides of a word on the board (prefix and Suffix) -