1. anxiety and excitation can lead to this. The first thing to try is relaxation. When you masturbate pay attention to the muscles in your lower abdomen tightening and practice relaxing this whole area while masturbating. This gets you in the practice of doing this during sex or other stimulating acts.
2. Low doses of SSRI (Selective serotonin reuptake inhibitors) can help with PE, there is actual research on this and the FDA has approved some SSRI's for this condition. The common ones prescribed for this are Lexapro, Zoloft, Paxil or Prozac. I beleive its either Prozac or Paxil that is FDA approved for this.
3. Cock rings can help but I would recommend doing a lot of research about this and to use one of the flexible silicon ones rather than the fixed rigid metal ones. You want to make sure you can get it off, the metal ones can get stuck on by the pressure and blood build up and the metal ones with releases can also get stuck. A silicon one can be pulled off or cut off in an emergency.
I would start with learning to relax and practicing that, and if that doesn't help then visit your doctor and consult with him regarding options and it also never hurts to get checked out medically, could be something going on that your not aware of.
Lastly don't ask how I know all this lets just say its related to my work. Hope this helps.