Back at AnimeNext, I mentioned offhandedly to fellow budding writer David S. McCrae "Any writer who wants to call himself one should have a fountain pen." (Paraphrasing from memory, but that was the gist of it.) I wasn't kidding then, and now I think I'm twice as not kidding.
When you write longhand, as opposed to typing, you force yourself to think twice as hard about every word you put down. You make the least do the most. This is not to say that everyone should ditch word processing in favor of longhand for first drafts — even if J.G. Ballard did that — but that taking the time every so often to write longhand reminds you, in the most direct way, of what economy of words feels like. I've gone back and edited the last sentence I just typed about three or four times, and I have to ask myself: how much more careful would I have been if I had been writing that out longhand?
Since few of us are keen on the idea of ditching typing entirely, my recommended exercise is either a diary or an idea journal — in longhand, and with a high-quality pen. I picked up a basic iridium-point and aluminum-barrel fountain pen from Muji in NYC for about $12; the ink cartridges that go into it are a standard-issue variety that cost only a couple of bucks for a pack of five or six. A journal with quality paper will run you about $10-15. Avoid paper of the same gauge that spiral-bound school notebooks use, or paper that is clearly high in pulp content or not far above newsprint. The ink will bleed through and the pen point — even a good pen — will snag and skip. Once you get into the habit of writing in a good journal with a qualitypen, it starts to feel less like an assignment and more like a luxury.
I know many people who resist writing longhand any chance they get, if only because they admit their handwriting is appalling. (I think penmanship was one of the first things to get dropped, before physical education, in many schools.) If they feel an exercise like this would be tantamount to self-torture, then by all means they should skip it. But for those who savor the contrasts for their own sake....