Next time you have an erection and want to masturbate, try this:
Don't take your underpants off. Instead, pull your cock and your balls through the fly opening in the front. Make sure your balls are completely out. Then, while you are stroking the shaft of your cock, pull up gently on the waistband of your underpants so that the fabric under the fly presses up on the swollen area under your scrotum. This will massage your prostate and feel really good.
You can vary the pressure depending on how you like it. I like to pull up hard just when I'm about to cum. It makes me shoot my load harder and farther.