I have received complaints and also experienced that the forums need u to log in everytime u enter after a long interval of time. I'm not sure if this has sumthing to do with the cookie settings, but I'll try to solve it out. In any case, try cleaning ur browser cookies, temporary files and offline files. Make sure u have cookies enabled.
Meanwhile, here's a solution I think wud keep u logged in the next time u enter. Make sure u had checked the 'Log me in automatically' box during ur first login.
Bookmark any random link from DPForums pointing to a certain topic, or forum, u can choose ur favorite one, or any other link like the one below:
Eg. http://dpforums.byethost14.com/dpforums ... m.php?f=29
Instead of visiting the main Index page everytime, visit this or any other DPForums link and u'll be shown logged in. It works for me, I hope it does for u.