This one is definitely better if you aren't wearing any clothes. Squat down on the ground and spread your legs open about as far as is comfortable. Then, lube up your penis, get into your favorite grip, and go. The key is not to "hop" up and down, but rather to leave your legs and thighs as motionless as possible. Focus the movement on the shaft and the head of your penis instead. Try some different grips – for instance, put both thumbs on top of your penis and two fingers of each hand below. Just keep going until climax, but be sure to maintain the motionlessness in the thighs and legs during ejaculation, as this greatly enhances the effect.