My penis is just under seven inches long, and this technique works amazingly for me.
I get a boner, and then I pull my boxers up so my nuts are above the waistband. I then sit down in a recliner or other sort of chair. I take my right hand and place it on my wiener like you normally would, but then I take my left hand and place it right under, and then move them up and down together. You either have to have smaller hands or a bigger penis to do this, but trust me, it's great. The great part is, my balls feel so good too because my hand squishes them on the way down and up as I stroke my cock.
I've found that the trick to great masturbation is paying close attention to your whole penis and your balls at the same time. Have your girlfriend try this technique out on you too sometime, she'll love it too