Depends on how and why your doing it
I'm not going to read all the replies before I answer. I haven't read anything in the Bible saying masturbation is a sin. The Bible talks about sexual immorality and lust and adultery, nothing about touching yourself in a sexual manner. I guess some will never get over the hangups concerning sexuality, as if it of itself is sinful. We are all sinners no matter what, that is why we have Jesus Christ. If what you are doing causes you to be immoral and your conscience( or the Spirit) tells you that you are being wicked, then cease and desist. But I think its better to masturbate than to burn with lust and go out and do something stupid, like having one-night stands or paying for hookers. Now THAT is sexual immorality.
If you can do it in a responsible and healthy and private manner, I don't see the wrong in it. Unless your wacking off to photos of kids or fantasizing about your brother's wife, etc etc. I think we all know when we are being ungodly.