Try to keep slang, and shortened words to a minimum. No one wants to read something serious only to find out the character's reaction was "Omg."
Other words include: LOL, ROFL, OMFG, SMH, JFC, etc. You get it.
It's short and supposedly convenient, but makes reading absolutely boring and meaningless. Try being more descriptive with emotion. Because that is a big part in grabbing a readers attention.
Speaking of using descriptive words, we all know this but, instead of using simple words like sad or happy, try using words like melancholy or jovial.
Ah yes, big boy/girl words are hard. But they will help in the long run.