This method is best used when you have a lot of time on your hands (no pun intended), as depending on what you use it can get quite messy.
When fully erect, but not necessarily fully hard-on, peel back the foreskin (if you aren't circumcised). Hold the base of your penis with the 'O' of an 'OK' sign made with the opposite hand you're going to use for the main stimulation, and placing the other fingers over your balls. Lubricate the hand you are going to stimulate with, and with an upside down 'fist' grab your glans. Make twisting motions, rather like driving in a screw with a screwdriver, but letting your hand slip; and with your index finger rubbing under the ridge of your glans. Make slight up and down motions if you like, just keep on going. If you don't feel the pleasure set in immediately, either grab slightly harder or make sure the back of your hand is facing you, the palm of your hand is rubbing the top side of your glans, and the tips of your fingers the 'sweet spot' on the underside.