If you're checking the box that says "Log me on automatically each visit", it should stay logged in. What can you tell me about what browser and operating system you're using?
I also find that the site remembers me until I try to post something. Is this what you're seeing?
You might want to try clearing your cookies and cache. There's a pretty good Google Article about how to do that:http://www.google.com/support/accounts/bin/answer.py?answer=32050