Actually, writing illegibly is one of the worst things you can do. The advice my powerscore instructor gave us was that basically, they probably won't read your writing sample, but most likely, they'll glance at it. And in that glance, there's a couple of glaring problems that could put you in not so favorable of a light. If it's scribbled and looks like your physician wrote it, it'll stand out. If you dot your i's with a little cutsy heart, and generally it looks like a 5th grade girl wrote your essay, it'll stand out. If you blatently use big words to impress, and then spell one of your 8 letter words wrong, it'll stand out. Spelling, yes, while a worry, may not reflect too poorly on you. They do realize that you've just taken an enitre lsat, you're still under time constraints, and your brain is mush. If you spell a nifty word wrong in the first sentence, however, it'll stand out.
So my strategy for the writing sample is to make it neat, try not to misspell anything, esp in the beginning or end of paragraph, and aim for at least half a page. I'll probably hit somewhere around the three quarters mark, but again, anything less than half will stand out.