This is probably obvious, but I think the most effective activity people can engage in to improve their own composition skills is reading. When you read well-written work, your understanding of language, style and tone, technical details (such as grammatical rules), and vocabulary will build. This widened understanding will inevitably inform your own writing. I'm not sure whether you're looking for tools to improve the technical quality of your writing or its content (are you a budding author?), but reading quality work will likely help in both areas.

If you enjoy genre fiction and are interested in using it in this strategy, I'd suggest going with older novels- on the balance, they tend to be crafted more finely than much of what is being published now- I think this is probably a result of the increase in how many books are published and the general decrease in the public's mastery of Standard Written English. It is also the case that books which are still readily available many years after their creation tend to be those which are good enough to have appealed to readers generation after generation. In case you're not much of a reader, here are some authors I recommend: In fantasy, think Tolkien, Le Guin, or Richard Adams rather than Brooks, in mystery, choose Poe or Doyle rather than Koontz or Brown, in science fiction, pick Orwell rather than Crichton, for romance, look to Austen or Bronte rather than Steele. Of course, none of this is to say that there are not contemporary authors who create excellently crafted genre novels (Le Guin and P.D. James, for example).

I doubt this is a particular issue for you (judging from what I've seen of your writing abilities on Q'land), but the other extremely important foundation for composition skills is a strong understanding of English grammar, which is something many of us were not taught as children. Some of the other answers have suggested various sources for grammar instruction. Your writing will become more fluid and expressive as you increase your understanding of sentence structure and the ways it can be manipulated.

This may sound silly, but I also want to suggest using prompts or giving yourself specific limitations when you do practice your writing. Oftentimes giving yourself restrictions to work within will force you to be more creative and exploratory in your work than you otherwise would. This can develop new skills, expose specific weaknesses, and force on-the-spot learning. There's a lot more incentive to seek out new knowledge or skills when we present ourselves with a problem to be solved rather than rely on a vague desire to expand our knowledge.

I hope some of that was helpful- I tend to ramble a bit in my own writing, as I'm sure you can see.

The Richard Hugo House is a center for creative writing that offers all kinds of writing classes. It is on 11th Ave, just across from Cal Anderson Park on Capitol Hill

http://hugohouse.org/

Do you mean the physical act of creating legible handwriting or are you referring to the compositional skills (grammar, syntax, etc.) of writing?

Do you know any other writers? Sometime a little critique group can help with this meet up every few weeks at a coffe shop, swap whatever you're working on and go to town on each other. Of you don't know other writers you could post an add on craigslist and see if you get along. Also try teasing your stuff to yourself out loud instead of just reading what you've written in your head. It can help you figure out what's reading awkwardly. Also dissect other writers that you like. Figure out what it is that you're attracted to in their language. Sometimes trying to write in someone elses style can lead you places you wouldn't expect. There is the strunk and white book ( elements of style I think is what it's called) that you can pick up used at most bookstores. It's thin but has common grammar, punctuation, etc to help with the bones of it. This time of year there are probably a bunch of copies since the semester is over for most schools. If you're within the Seattle city limits you might try checking out http://www.826seattle.org/
It's a non profit started by Dave eggers of mc sweeneys fame and it's a nation wide writing torturing program. They usually have workshops and what not. The Seattle one is in phinney ridge area and also has a space exploration store connected to it that pays the bills to keep it running ( San frans is a pirate store, new Yorks is a superhero store). It's mainly geared twoward matching up writers with kids to help their writing but you could go and meet some of the volunteers and see what resources they would recommend. Also check out artist trust on Capitol hill they have tons of resources in their library and know of local work shops. If you get to the point where you're interested in publishing they recently added a writing track to their EDGE program that teaches business skills to artists and how to navigate the stuff they didn't teach in school or has changed ( with the digital age) they also host workshops for various disciplines as well as how to apply for their grants and programs that cost money to help you have the best possible application and het fellowships for their programs and other orgs programs. Good luck