Two delightfully helpful answers there.
If you are not alone then obviously a willing partner who will just tie you down and wank you fast and hard until you cum, and then do it again for teh lulz.
If you are alone then taping a vibrating egg or fake pussy to your cock then tying yourself into some timed self bondage.
I inadvertently forced myself to cum just like that... The plan was to tie myself with a vibrator strapped to my cock and be teased like mad while my pc played a slideshow of images, it was going well until randomly, out of nowhere my cock spasmed and that was it... from there is was all uphill... lucky the bondage was pretty poor so i escaped before the torture began