Answering this question meaningfully depends on your skill level. There are some very basic things everyone must know, which can be found in the Beginner's Guide linked on the home page. From there, I'd suggest you focus on three key things:
1) Develop your vocabulary, which is best done by studying the 3- and 4-letter wordlists and by playing opponents who are better than you.
2) Think and play defensively. Know the types of board setups that can result in big scores (e.g., lanes to TW, vowels next to DW or TW squares); then avoid opening those opportunities unless you're scoring big or have another strategic reason for it. That is, be a stingy player!
3) Make bingos a part of your strategy. That is, learn to use all seven letters in a single move. In most random games this strategy doesn't matter. However, elite players use bingos for long-range hits on high-scoring squares, while cashing in on the 35-point bonus.
This advice is quite generic, and it would take a lot more space that this forum affords to delve into the techniques of a great strategy. But I hope this helps a bit.