In live chat, here's what you can get away with.
- Not capitalizing the first letter of every sentence. (not for every sentence, though! just little add-ons. see what I'm doing here?)
- Not ending bland statements with periods. ("ooh, creepers") However, don't do this too often.
- You can split sentences into multi-line clauses, as long as they're fairly long. (not on forums, though. only live chat)
- ALWAYS SPELL PROPERLY!
- Always capitalize "I". Even if the first letter of the sentence isn't capitalized.
- Mixing up "to" and "too" and "their" and "they're" and "there" makes you look absolutely retarded.
- Anyone who Types Like This With Bloody Capitals On Every ****ing Word is unforgivable and I hate them.
I often do many of these in live chat. If you have a habit of not ending short sentences with periods, or some other form of ending punctuation, Just add an emoticon. It makes things seem more friendly ;D
Though if you want to be taken seriously, #4 and #5 are a must, in my opinion.