Ok, first I gather what I'm gonna need... which is a pair of panties (silk, lace are best), some lube, and the anal probe of your choice. (I use a small glass ornamental bottle-cos it starts narrow and then gets bigger, which feels great.)
When you have all the stuff, go into the shower with it all, but don't put the panties on yet. Turn on the shower to a lukewarm temp, and then get under it and gradually reduce the temperature while stroking your dick. Sometimes I can get it all the way down so its straight cold water, but on other days I keep it slightly less cold.
When it's as cold as you can possible bear, turn off the shower and put the panties on. They will get really wet and stick to your cold, hard cock, which feels great. Next turn the shower on, and now make it as hot as you can bear. Then sit down in the shower, and slowly raise your legs so they are against the wall, and you are on your back with your feet in the air, and the hot water splashing on your stomach.
Now pull down the panties so they are round your ankles, and start working on your ass with your plug/bottle/whatever and lots of lube. You have to make sure your ass is at the right angle so that the water doesn't run over it, otherwise it will wash away the lube and make things uncomfy. Once you have the bottle in your ass, slowly work it in and out while pumping your dick... until you cum.
I have achieved the best orgasms ever in my life by doing this... and believe me I have tried most the techniques on this site.