What tips do you recommend for improving one's writing?
Admire the craft
While the best way to do anything is to practice, it's also important to practice correctly. I'm not sure how many years of writing experience you have, or what kind of writer you want to become, but improving your writing skills will help you in many ways, not just expressing yourself on page.
The absolute best way to improve your writing is to read a lot. Through reading you will learn the mechanics of sentences and paragraphs, the elements of a plot, character development, and proper grammar usage. The best part about this technique is that you'll learn without realizing it. It works on the same principle as going to a foreign country to pick up a new language. By submersing yourself into good writing, you'll come away with better skills. Read from many different authors. You'll find that if you read several books written by the same author, your writing style will start to sound like theirs. It's not a conscious thing, which is one of the beauties of this technique. After reading Chuck Pahalniuk (Fight Club) you might find yourself writing shorter, more declarative sentences, so switch it up and pick up a Dean Koontz novel for more lengthy, descriptive sentences. Then write. If you like to write stories, then go for it. If you like to write articles, or hubs, or even blog, it'll all work. You can only get better.
There are many ways to improve your writing. One simple way is to use literary devices in your work. A short list with examples follows.
Parallelism-The use of identical or equivalent syntactic constructions in corresponding clauses or phrases.
For example: "You had Dalton fooled. You see, he was rather fond of you-thought of you like a pet, really. His favorite spaniel. He believed he had you under his control. I know he let you get away with running your mouth like a spoiled child, or faking your strength. But you showed all your cards the other day with your little stunt. You're a very powerful tool, which you know. You fooled Dalton, but you won't fool me."
Alliteration- the repetition of a leading consonant sound in a phrase. DO NOT OVERUSE!
"James just jumped to conclusions, leaving little doubt of his state of mind."
Metaphor- a figure of speech in which a term or phrase is applied to something to which it is not literally applicable in order to suggest a resemblance.
"He is the rock upon which I stand."
There are also a number of habits that you should avoid.
1. Stop writing informally, even in an informal situation. So if you're writing to someone through AIM, MySpace, a blog, or what have you, do not resort to LMAO or LOL, or any other silly shortened phrase. Get creative. Think of new ways to express yourself other than using a clichéd lol.
2. Stop leaning on verbal crutches. Speak with more grace and pride. You will find that the more you read and write, the more articulate you will become, and vice versa. Instead of filling your brief silent moments with a verbal crutch ("um," "like," "uh," "you know,") just stay silent and think about what you want to say. This will force your mind to think, rather than saying "Uhhhhh," as you come up with the next word.
3. Do not over-use certain words. There are many words for the same thing. The easiest way to bore a reader is to repeat the same word in a short amount of space, so mix it up.
4. Avoid clichés when possible. They're boring and unimaginative, so by avoiding them you'll force your brain to come up with something original.
5. Use exclamation points sparingly. Most of the time a period will work nicely.
And your on your way to becoming a better writer.