I hate to admit it, but anal stimulation is the key. I'm a straight male and it took me a long time to get comfortable with the idea of having something in my anus, but trust me it is worth it.
I've found that I using a modified 'butt plug' can produce some really intense orgasms. Here we go... Get any size 'Jelly like' butt plug. (I've found that a medium/large sized one works best, too small and it won't work here.) Cut off the 'T' part that would normally keep the plug from going in too deep. Insert the 1/2 a plug upside down (with the newly cut part going in first). Let it go all the way into you. Now squat down and sit on your ankles (or anything to make the plug want to come out with gravity). As you relax your anus the plug will start to come out as the tip pokes through. Since your anus isn't really relaxed or 'open' you will feel it stop when the plug reaches the thicker part (you could push it out, but it won't fall out). As you begin to masturbate, keep squeezing your anus closed every time the plug pushes out, this will send it shooting back into you, pushing past your prostate. This constant open/contraction of your anus feels totally amazing....you almost lose track of what to concentrate on more!! I've actually had an orgasm from this without ever touching my penis....Trust me, some of the best orgasms are from doing this! (Hope it makes sense.)
[Webmaster's Note: This sounds like a good way to end up in the hospital in quite an embarrassing situation. The 'T' part is there for a reason... If you cut it off, it could be extremely difficult to get it out. It would be better to find a toy with a string attached to it-they do make them.]