There's no way of making it permanent at the moment Poppy until Andy has it completely fixed.
All I can suggest for the moment is to log yourself out using the instructions, then clear your cookies and temp files, close the browser, re-start it and then try logging-in using the login at the bottom of the forum index, see if that solves it for you.
BTW, is there a reason you need to be logged out at the end of each session?