Firefox has in-post spell-check, which I abuse regularly. Usually, I'll just hurry up and hit post right after the words are down, but I tend to read through my post anyway, and I'll usually Edit-away the errors I run into during the post-read. Usually pretty functional, but sometimes I'll miss something, or typo in an odd, non-red-lined way, and never notice it. (More and Ore was something I did a few days ago, took me a few hours to notice.)
Assuming it's a long discussion I plan on vesting time in, I'll scroll about finding people, quoting them, addressing their post, double spacing, then quoting the next guy. Rinse repeat until done. (Also, as soon as I hit the quote button, I usually trim down the quote to clean up quote towers or trim their post if it's long, for cleanliness's sake.)
Then post.
EDIT
And like any person, I tend to remember something else to add after I post, which is why I use this "EDIT" format.
When I'm writing reviews, I will keep preview open constantly to check my image tags, format spacing, and how things will look in the actual post. Although, writing reviews and rants usually is a pretty big process for me, which is why my reviews especially always look so attempted-professional.
No one look at my PSU review, it is the most cleverly-arranged thing I've ever done, and scary as hell in size and scope.
