Scrotal tying. Yes, I've done it, but not with a partner, only in solitary play. I dont fool around too long in this position, because it seems to restrict blood flow to the sack, and definitely cools it down.
I tie my sack off above the testicles, using the heavy rubber band that supermarkets use to wrap celery.
First time I did it I was a little freaked out. Damn thing was so tight I knew there was no way I'd cum. And it wouldnt come off as "easily" as it went on, lol. To remove the rubber band it is necessary to CAREFULLY slide the blade of a scissor between the band and the scrote, and cut it.
I was reluctant to masturbate to completion with the band in place, since I dislike retrograde orgasms. But, when I finally came, wow. What an orgasm. No retrograd component at all, but the constriction limited the amount of semen that accompanied each pulse of my penis. This did not decrease pleasure, but had the effect of lengthening the duration of the orgasm.
You want those heavy rubber bands from the produce section, the kind where you look at them at first and think "Nah, no way jose." For your first time, keep scissors nearby in case you freak out when it first snaps on. Slide the blade in FLAT, then once under the rubberband, rotate it so the cuttting edge faces the band. Expect it to come off with a sharp snap.