Have you tried writing as a way to explore your thoughts? As it doesn't involve your master, or masturbating, it might help a little in releasing some tension. Perhaps more so if you know your master will look over what you have written before he deals with you next (and hopefully will get some inspiration from your fantasies).

...alternatively, for a bit of physical relief, how about doing some ridiculous form of exercise - tire yourself out with a massive run/do some kickboxing/martial arts etc.