I used to be a cop and my male partner would tell me that as soon as he felt the end was near, he'd think of .............. dead puppies..... that's so gross huh, but funny!!!!
Glyttch covered a good topic about being in good physical health, the better the physical health you have the better sex life you will have. Also there are topical solutions you can apply to cause numbness, works for some but not all.
Try to keep the romp up with varied positions, constantly changing it up until you feel satisfied enough to finish. Mental focus is everything, dead puppies is extreme, but I've had partners who swear by thinking about sports????