If you look at a post you made, then somewhere (I think it's the upper right corner of the post itself) there's a link that says "Modify".  Click it, and it opens up your post for editing.

I tend to hit "post" before I proofread, so I tend to use that button a lot!  If folks have already responded to some mangled post of mine I'll use liquid paper strickethrough so that folks know what I've changed.