With the rising cost of writer's retreats and the sadly decreasing frequency of them you may not have a chance to attend one to work on your craft. Don't let the cost of flying to some distant location or the retreat fees deter you. There is another way. Either with a few friends or even on your own, you can create your own writing retreat. Often we don't get our writing done just because we don't have the time or lack of distractions. Think of it as a vacation, make the time.
The key is to get out of your regular environment where you're tempted to do other things that clamor for your attention. If you can get away for a short time on a daily basis do so. But if you can only do it on a weekly or even monthly basis that's great too. Any time truly alone with your writing is quality time. Your local bookstore or coffee shop are perfect for short getaways.
If you have a few writing buddies you can create a more in depth retreat. Just last February my Scribe Sisters got together and worked on our novels together for a week. Though it would be a blast, you don't have to go to an exotic or expensive location to do this. Pick an inexpensive bed and breakfast, or meet at a friend's house that will be empty for the duration of the retreat. In our retreat my critique group and I worked on our inciting incident chapters, our plot point 1 chapters, our mid point chapters, plot point 2 chapters, and the climax of our novels. To see how we conducted our own retreat check out these posts from it: http://heathermccorkle.blogspot.com/2010/02/first-day-of-writers-workshop.html
The important thing to remember is to have a plan for working on your writing and to stick to it. Whether you're working alone or with a group, have an hour, or a week, its the commitment and the desire to improve that is important.
For a few books that you can use as guidelines for you own retreat workshoping check these out:
Writing The Breakout Novel by agent Donald Maass http://search.barnesandnoble.com/Writing-the-Breakout-Novel/Donald-Maass/e/9781582971827
The First Five Pages by Noah Lukeman: http://search.barnesandnoble.com/The-First-Five-Pages/Noah-Lukeman/e/9780743290937/?itm=1&usri=The+First+Five+Pages
The Fundamentals of Fiction by William Bernhardt: http://www.williambernhardt.com/dvds.php
And check out this great article on Do It Yourself Writing Retreats by Writer's Digest: http://writersdigest.com/article/Do-It-Yourself-Writing-Retreats/