I just save them as text files in a folder on my harddrive and then paste them into the lj editor (I write my posts in the html editor, but you could save the rtf version as rtf files) This doesn't save stuff like mood, comment settings etc but that stuff I'm happy to do on the fly.

The main problem with this is that if there's a lot of complicated formatting (like polls or lists) it can be a pain to preview them in lj then save them back in the text editor. But most of the stuff I save as drafts is pretty basic html and it doesn't take long to fix any bugs in the lj editor before posting. I used to use a client which saved and previewed my drafts but it stopped working and I never bothered getting another one.