Masturbation is never specifically declared a sin in the Bible. 'Sexual Immorality of all kinds' is to be abstained from by Christians - but to say that Masturbation is therefore a sin is begging the question.
Lust is always a sin.
So if you're masturbating without lusting you should be fine.
Source: I've read the whole Bible.
/edit: I forgot! If you're a Catholic, Church tradition and not just Biblical interpretation is the key to understanding a moral issue. Church teaching has always been that Masturbation is sinful. Therefore, as a Catholic, masturbation is a sin, regardless of what you think the Bible has to say about it. That's just the way that it is. Furthermore, as a Catholic, masturbation is a Mortal sin - meaning that you must participate in a proper confession in order to have this sin absolved. Mortal sins such as masturbation cannot be purged in purgatory - if you die in a state of Mortal sin you are destined for eternal damnation.
Source: I've read a fair amount of the Catechism and nearly joined the Catholic Church. I didn't join though.
