Just as you can rotate your desktop wallpapers periodically with tools like Wally, Personas skins can be periodically swapped with Personas Rotator, a firefox extension that automatically rotates and loads your Personas skins. Loaded skins are chosen from within collection of your choose, you can log into GetPersonas.com and select your own favorites, the add-on will try to rotate your My Favorites personas. If you have no My Favorites personas the rotation is completely random through different collections.
After installing the add-on you can modify the default one minute interval to say 30mins by clicking on the Personas icon at the bottom left of your Firefox browser, in the menu take, Personas Rotator -> Options. Personas Rotator is a free download that needs both Firefox and Personas Plus to work.