Frequent play to your current tolerance will do as much for you as anything else. Frequent meaning at least twice weekly. Just as with any activity that you must have endurance to accomplish, repeated practice at your current limit will help you to accomplish more. Don't attempt to push past your limit every time. Just find it every time, and don't exceed it. His judgment will soon become the deciding factor as your mind begins to withstand more than your body will allow.

Another thing you will find helps you. Letting go. You must find your spot in your head that allows you to let go, trust your Dominant not to kill you, and just enjoy the sensations you feel.

Add to the pain you are being given interspersed with reasuring caresses to "make it all better" and your mind won't associate it with pain as much. As the endorphines roll out, your mind will confuse the pain within your threshold with pleasure.

Most of all, make it fun, not a challenge.