Then and Than.
Then talks about time.
Than is used when comparing.
I usually proofread prior to posting and correct anything I find incorrect. It's the receptionist in me. However, there are times where I have found mistakes after I posted and I'll just hit the "Edit" button and correct it. If I'm not sure I'll put "SP?" right after the word.
When I went to business school, I learned to type and proofread. We were told not to rely on spell checker b/c the word could be spelled right, just not used correctly.
The instructor also told us that it is a good idea to have someone else read over what you typed. While you are proofreading, you can see a word spelled correctly when actually, it's spelled wrong. Which is what happened to her one day.
She was trying to spell the word "assets" but forgot to type the "t". When she read it to herself, she saw "assets". When she had someone else read it, that person immediately caught her mistake
. Good thing, b/c the document the instructor typed was a legal document 
.