You could try writing your longer posts in a text editor (TextEdit, MS Word, OpenOffice, Etc) first. You can write as much as you wish, save as often as you like, and even spell check. Simply copy and paste when you're done.

Though, if you're like me, you don't know which posts will be the long ones until it's too late. I don't suppose you could simply hit the "BacK" button on your browser and see the text you typed again? That works on the browser I use (Mozilla 1.5) even if the page is php, javascript, or pearl generated.