Premature ejaculation can be / is caused by the over contraction of smooth muscle tissues in/around your pelvic floor. Aka PC, BC, etc. muscles that you guys refer to them as. This is normally caused by too much calcium in the body. Magnesium is required for muscle relaxation. Most Americans have way more calcium in their organs, tissue, and cells than necessary, and it causes a long, long, long list of issues from premature ejaculation to deathy by some sort of vascular disease in the long run. Please refer to a thread I just started and look into magnesium to counter an over abundance of calcium if this is the case.
More than half of the US population is mg deficient due to having way too much calcium, stress, carbs, etc. in their life/ diet. There is some useful info that can be found for the relation between mg and premature ejaculation. It's very difficult to study and verify magnesium deficiencies because only 1% is found in your blood. As long as you don't have liver disease, by all means give a supplement a shot. Any excess will pass through you in the form of number 1 and if way too much via number 2. Never take more than 800-1000mg/day. Avoid the oxide form of mg as it is known to cause diarhea and any residual amounts can be neurotoxic.
Also, if you are a smoker and don't supplement with mg, you are mg deficient without a doubt to some degree. If you are a long-time smoker of 30+years then your atherosclerosis (impotence/ejaculatory control) should somewhat improved with mg. Obviously, quitting smoking should be solution attempt #1.
If you are a smoker and deal with any sort of sexual ailment, it's your fault because you smoke. If you refuse to believe this then you're on your own. Continue to pursue all the wild PE, or pills or whatever else you can find to pretend that quitting smoking is not the solution.
There is also such a thing as over-stimulation. If you play with it too much the nerve endings will become less sensitve (not permanant) and the pelvic floor/pubic smooth muscle tissue will be exhausted and prevent you from having control. Let go of the joystick for 48 hours and then check back. If you can't have the self control to do this then evaulute why you are such a weak-minded individual.