Good answer R2R.
There is also this answer too, taken from a ministry site.
"'When a man has an emission of semen, he must bathe his whole body with water, and he will be unclean till evening. Any clothing or leather that has semen on it must be washed with water, and it will be unclean till evening. When a man lies with a woman and there is an emission of semen, both must bathe with water, and they will be unclean till evening. (Lev 15:16-18)
Notice verse 18 deals with seminal emission during sexual intercourse, but verse 16 is speaking of a seminal emission that does not occur as a result of intercourse. If this is indeed a reference to masturbation, the matter-of-fact way in which God deals with this matter certainly indicates that no feelings of shame should be attached to it. He simply said, "Go take a bath."
It seems to me that if God were really bothered by such a practice, then He missed a teachable moment here in this passage. Since He clearly condemns sexual intercourse between men in Leviticus 18:22 and says "that is detestable", I think that if He had really wanted us to avoid this particular practice, He would at least have said something like "That is disgusting" instead of "Go take a bath."
Masturbation has been around for a long time, and since God does not clearly condemn it, I would not be too bothered with it either. Masturbation is practice far more than adultery or fornication yet God is practically silent on the issue. This ought to tell you that God is not overly concern with it.
However, let me caution you against addiction to masturbation. Just like most things, masturbation can turn into an addiction. If you find yourself constantly—several times a day—masturbating then you can be reasonably sure that you have a sexual addiction. Paul said, "'Everything is permissible for me'—but I will not be mastered by anything" (1 Cor 6:12). This includes masturbation. Also, you should never use pornography to masturbate, and if you are married, keep your thoughts on your wife. It is the same sin to make love to your wife while thinking about another woman as it is to masturbate while thinking about another woman. Concerning single people, I have no advice other than a prohibition to pornography.
If a believer uses masturbation to alleviate sexual temptation that's far better than actually being tempted to commit fornication or adultery. I would rather have a man masturbate than go to a prostitute.
Another thought, if masturbation is sinful, then you would expect there to be bad health consequences to it, such as found in adultery, homosexuality, and fornication (diseases for one thing). Instead, research has found that masturbation serves to release sexual tension. If a man is prevented from having sexual release, then the seminal fluids can back up, causing increased sexual tension and physical discomfort. It also increases the risk of prostate inflammation. This might explain why priests who practice celibacy and who view masturbation as sinful have a higher rate of prostate cancer.
Since there are proven medical benefits to masturbation, it is difficult to make a biblical and reasonable case against it.