This might help
http://www.youthspecialties.com/article ... bation.php
I'm still a noob so don't have anything to contribute, but i'm cyphoning off others research on the net.
SO if i can keep my mind totally clear of any thoughts/images (Which i have mentally trained myself to do) it's not a sin? SO FAR to my understanding it;s a sin because of lust looking at women and thinking sexy thoughts about them, and adultery of the mind.
And also, every now and then 'images' creep into my mind, but just for a split second, it's ok if i push them out of my head right? It's only a sin if i willfully keep the fantasy going right? Someone help i've been dwelling on this for a few hours.