Dumb Little Man gives seven tips for writing (online). Number 2 always needs my attention:
Short Sentences and Paragraphs are Best
Online, it’s hard to read long paragraphs. Break things up: write in shorter paragraphs and shorter sentences than you normally would. If you want some great examples, take a look at any post on Copyblogger.
If you naturally write in quite long paragraphs and sentences, just do an extra edit to split these up. It only takes a couple of minutes, and it can make your blog post (or Facebook update, or email) much more readable.
Shorten. Those. Sentences... Tips 1 and 5 are great too.