1. Always look to see which colored tiles are reachable.
2. Always check every word on the board to see if one letter can be added to the front or end of it.
3. Always check before you finish your play to see what you are opening up for your opponent.
4. Make plays parallel to words as much as possible so you get scores going in several directions.
5. Don't assume your opponent can spell, won't try to fake a word, or won't challenge you.*
6. Try to play people that are better than you as much as possible.
7. Don't complain about your tiles unless your opponent complains first. Let him/her start the whining. If you have poor tiles, either swap or shut up. A good player can tell when his/her opponent is working with crap.
8. Between games read a lot, and have a dictionary on hand to look up words. You can never have a good enough stock pile of words.
9. Big is not always better. Short words with big scores are just fine. Impress me with the bottom line--unless you use all your tiles and get a BINGO.
10. It doesn't hurt, especially at the end of the game, to keep track of which tiles have been played and what letters are left. In a close game, it can make the difference between a win or loss.
11. (A bonus tip!) Never assume in the first two-thirds of the game that you are too far ahead to be beaten. Cockiness leads to less aggressive playing. Next thing you know, you'll be scrambling to stay on top.
*Does not apply to the facebook version of Scrabble, which is why I really hate it. I also hate it because it accepts lots of phony words like XI, QI, and ZA to make people who are lousy players feel better, and to make good players lazy. Did that push anyone's buttons? (Hope so.)