It depends for me. If I have a really long and exciting scene in the works ( Red, think Hotaru vs Geara in EE ), I type it out on another document, adding to it over several days. Then I finally submit the final thing.
Most of the time I'm just writing right out of my head when I post. I always have segments of the storyline already planned ahead of time, though I'm just making up the in-depth details on the spot.
( Btw, Red, there's a chance I might not be able to post in Ryuketsu until about next week. >_< )