Preventing high score cheating in games

I’ve been trying out different methods, but sadly none of the ones I’ve implemented really work properly (one involved storing a text file with a key in on the server and having Flash load it while a .htaccess file prevented any other access to it, except Flash is run from the browser so it wouldn’t be able to access said file either).

Are there any good techniques out there for stopping people simply reverse engineering your Flash files and then submitting high scores of their own choosing?