You're not about to affect your testosterone levels by masturbating.
Testosterone is one of a handful of hormones your body uses to regulate sexual functioning. Testosterone is a factor in sperm production, for example, and in the development of secondary sex characteristics (like a sharper jawline or a deeper voice and broader shoulders). Don't be led to believe that since you will be producing more sperm as a masturbator that you'll over- or under-compensate for this in your testosterone production. Your brain will do a dandy job all on its own telling how much you need and how much to make. The only way you can really handicap yourself is by what you put into your body. That being said, aim for good nutrition, and avoid the use of anabolic steroids (which act like synthetic testosterone). By putting steroids in your body, your brain detects their elevated levels and figures that to retain normal levels, it has to decrease its own natural testosterone production. Because of this, the steroids injected do what little they can to help (not the best because remember they're only synthetic) and you lose out on your own testosterone factory. In the end, you hinder those activities your body naturally uses testosterone for (and really, no one WANTS their testicles to atrophy, do they?).
P.S. Biology/physiology/kinesiology is my field of study.