Alright - I'm going to assume you aren't married. I do not believe the act of masturbation is sinful, however I believe the thoughts that accompany masturbation
are sinful.
Check this out -
Matthew 5:27-30 "You have heard that it was said, 'Do not commit adultery.' But I tell you that anyone who looks at a woman lustfully has already committed adultery with her in his heart. If your right eye causes you to sin, gouge it out and throw it away. It is better for you to lose one part of your body than for your whole body to be thrown into hell. And if your right hand causes you to sin, cut it off and throw it away."
Jesus said that. Notice the parts of the body He mentions - your eye and your hand. Understand that Jesus understood this struggle, but do you believe Jesus ever gave in to the temptation to touch? Of course not. We know He never sinned.
If you are enjoying sexually explicit thoughts about a woman that is not your wife then I believe it is sinful. It is a completely natural act, but to live by the carnal and fleshly nature is to be no different from an animal. Live by the Spirit, not by the flesh!
Hope this helps and God Bless.
-Wes