In girl-on-top, as you are sliding up his penis, contract your vaginal muscles, squeezing your way up his member. Then release, and slide back down his penis. The clenching of the muscles will help you reach orgasm and give him a tight fit.
In missionary, up the G-spot action by having him rotate his body clockwise, using his arms to support himself, until you’re facing opposite directions. From there, grip his waist with your thighs as he thrusts back into you.
As you feel yourself reaching orgasm, slow down to an intensity of about 75 percent of your full speed, stay there for one minute, and then pick up the pace again. This technique will make your orgasm even more intense.