Ha! I was looking for this as well. On some machines I am always logged in. On my laptop I have to log in every time. (the Horror...one more click) I "think it has to do with the first time you log in and it asks if you want to remember the log in stuff. I may have skipped over that part on my laptop.
I havent done this but possibly delete all your cookies. Or find your BH cookie. then re log in...and set to remember your log in info. I haven't done it because if i delete my cookies I have to log in all my other sites