In Scrabble and Words With Friends, the vast majority of words that you will play will be of three, four, or five letters. Even though this is the case, it's crucial for your success in these games that you have the entire list of 107 two-letter words memorized. Even if some of them are usable only in either Scrabble or Words With Friends, but not both, it pays to have them ready to go. It's a simple matter to remember which ones are which if you have all 107 committed to memory. The point of playing words of three, four, and five letters is to get better letters to make bingoes. Keep a words by length record of your successful plays.
Vocabulary is a strong asset in these games, but the key to victory is maintaining rack balance. Here is a question for you. Would you rather play syzygy on a triple word score or play raisins on a double word score? Syzygy scores 25 points, counting putting one of the Ys on a double lettter score. Raisins scores only 7 points. If, however, you happen to put both the R and the ending S on double word scores, then it is worth four times the normal score. Sure, syzygy is cool, and all, but how often will you have both Ys, the Z, and a blank available in your rack? By knowing your list of words by word length, you'll build towards better plays all the time.
Raisins can be easily made by maintaining proper rack balance. That means you keep the letters R-A-T-E-S in your rack if at all possible. Secondary to those primary letters, you also want to keep I-N-L in your rack. By doing this, you will be able to work towards better-scoring words by length up to and including bingoes.
It is an important strategy to note that it is sometimes better to take a lesser score to maintain rack balance. Let's say, for example, that you have the following rack: X-E-T-H-E-P-W. There is an open S on the board, and you can play hexes on a double word score. That's 16 points, but it leaves you with T-P-W! Yuck. You can also play phew for 12 points, which leaves you with T-E-X, which is much better.
Imagine if you pull T-I-L-E from the bag when you replenish. Now, you can play textile. That's a minimum of 14 points if you don't touch any premium squares. If you happen to get the X on a triple letter score, suddenly that's 30! That 16 doesn't seem so good now, does it? The words length was seven letters, so you get a whole new rack to balance.
With your words by word length list in hand, eventually, rack balance will become second nature to you. Then, you will see your scores and winning percentage go up and up. Remember to sort your best plays by word length. It's easier to remember them that way. The other tactic that is important is not to leave your opponent an easy place to play high-scoring words, especially bingoes. Take a lower score to block your opponent. If you can maintain rack balance at the same time, all the better!