I am far from being an expert on the art of writing, but I have achieved a measure of professional and financial success through my efforts at stringing words together. With that caveat out of the way, I want to add to some earlier ideas I shared on how to improve your writing skills.
Write every day. The only way to get a novel, research paper, or dissertation completed is to plug away every day. Even if you get nothing more than a sentence or a paragraph completed, you have made progress.
Get critical feedback on your work. Even the best works of literature go through editors and readers prior to hitting the bookshelves. As a writing tutor I found that some of the best writers I encountered were those who came for additional help. Find people who will constructively critique your writing, and avoid those people who say something like "It's great!" Every piece of writing can be improved upon.
Regularly use style guides and grammar tutorials. One of my favorite sites is the Online Writing Lab (OWL) at Purdue University. It never hurts to polish basic skills, such as the rules on commas or apostrophes.
Have a dedicated area for writing. This should be a space free from major distractions where you are set up to work. Make sure you have good lighting, a comfortable chair, and room to pile up papers and books.
Teach your family and roommates that writing time is your work. Sometimes people assume that you are just playing, and do not realize that you are working on a writing project. Be clear about the seriousness of your work, even to the point of putting up a "Do Not Disturb" sign.
Buy foam earplugs. These have been a lifesaver for me, coexisting in a house full of teenagers. When I wear these I am much less distracted by sudden noises or kids arguing about, say, TV programs.
Totally kill virtually every adverb completely from your writing immediately. Adverbs should be used with caution, and resist the urge to string together two adverbs in a row.
Music and writing may not be compatible. This, of course, is a matter of preference, but I cannot write if I am listening to music with lyrics. I do find some classical, jazz, and folk instrumentals conducive to the writing process. Experiment!
Write when you are most productive. The best times for me are early morning and late in the evening. I try to avoid writing between 2:00 and 4:00 in the afternoon, when I am usually in need of some physical exercise after the stresses of the day.
Use works of great literature as models of style. One of my favorite writers is Herman Melville, and I sometimes pick up one of his books to study a few random paragraphs. Watch how a skilled writer builds complicated sentences and plays with words, and learn from the masters.
When you are in a writing groove, make the most of the moment. Today was not very productive for me (with the exception of this post), but I think I cranked out twelve pages of text on Wednesday morning from 10:00 am to 1:00 pm. On those days when the muse is missing, just push yourself to meet that one-sentence, one-paragraph, or one-page goal; when you do get inspired, ride that metaphorical Palomino until it gasps for air.
Finally: I have met some excellent writers through the Internet, so feel free to chime in with any tips you might want to share. I am always looking for new ideas on the art and process of writing.