The Grammar Girl posted a style guide for writing for Twitter. Since you only have 140 characters to work with, you may need some guidance on how to get the most out of this limited writing real estate.

I especially liked this tip:

Don’t use abbreviations such as 4U and L8. They make you sound like a 12-year-old (which is bad, unless you actually are a 12-year-old)

Yeah, not cool. I remember seeing someone do that in a response to me and I had to do a Google search on some of the abbreviations he used. Totally wasted my time which is NOT the point of Twitter.