I've had that sort of problem on several forums. Honestly, I'm in the habit of always copying what I've written before I submit it at this point (although I stay logged on permanently here, so it hasn't been a problem) and sometimes I'll compose posts in a word processing program, just to make sure I don't lose them. I also recommend Firefox, because I've had good luck with the back button bringing me to my "post reply" screen, with my post still intact.