1. Despite the name, it is not a game of word-making. It is a game of strategy that happens to involve words. A good vocabulary is important, but no more than height is important in basketball. Just like Shawn Bradley and Gheorghe Muresan weren't particularly good basketball players, a "tall" vocabulary does not assure success in Scrabble or Words.
2. With that said, to become elite, it is mandatory to know all the two-letter words. From there, one should learn all the Q and Z words (within reason ... you're probably never going to play "benzaldehyde") and then the J words. Then learn as many three-letter words as you can.
3. On that vocab alone, you WILL beat 85% of the people you play. With experience, you'll learn how to utilize your small words. It seems simple, but with three tiles, you can make three or four words, which racks up the points quicker than most people realize.
4. In the same vein, learn from people you play. I learn new words all the time from other players. "Chaleh" is the most recent example, but it happens quite often. Don't think, "That guy is cheating." Even if he is, he is short-changing himself and teaching you.
5. Once you start to get a feel for using the short words to your advantage, the next step is to set up your rack. Naturally the easiest bingoes to make come when you have an "ers" or "ing" rack. When all else is equal, a 14-point play that leaves you "ters" is better than a 20-point play that leaves you "tkur."
6. You'll figure out the true evaluations of this yourself, but my general rules for an S or a blank is that I won't play an S unless it gets me 10 more points than the next best play and I won't play a blank unless it's at least 12 points better than my next best play. Obviously, if you have 3 S's, this changes, but it is nearly impossible to underestimate the true worth of these tiles.
7. Play defense. Of course, you never want to leave your opponent the TL/TW combo with a vowel sitting in the middle. That's asking him to drop a 60-pointer on you. But also, if you get out to a quick comfortable lead, muddy the game up. V's and C's are your best friends for this purpose because they are the only letters that do not have a two-letter word play off them. With experience, you will learn to leave un-bingoable boards, or at least close to it.