I use a FireFox plug-in called Lazarus. Solves this problem fairly nicely for many sites, since most of them don't squirrel away unfinished form submissions.

I also tend to edit substantial posts in a real editor. So the browser crashing or even the editor crashing doesn't mean I've lost the composition. I've also heard of a browser plug-in that makes such happen automatically ("vim everywhere"?), but I've never tried it (cut'n'paste isn't terribly hard).