I also want to mention that I always wait 15 seconds or so, before clicking anywhere to leave the Draft edit page. I figure that if I leave the page too soon, it will not have saved. I'm sure 15 seconds is more than enough time.
I thought Mark added something (a few months ago) that would show when a draft was being saved, similar to what Gmail does. But I don't see that anymore. If it was there, it might tell you if your drafts are not being saved promptly.
Andy