Dragster,
If you haven’t already, maybe you could try these:
When having sex missionary style, deliberately rub your pelvis in a way that the pad of fat around your dick will rub her clit.
When having sex (especially doggy stye) don’t insert your penis all the way. Insert the head and nothing more than just below the head. I haven’t found a girl that doesn’t like this. That’s not to say to never completely penetrate, you should; just don’t completely penetrate all the time. A pussy likes to be teased, my friend. Besides, the most sensitive portion of the vagina is right around the orifice and a man’s glans can do some pretty nice things to that sensitive area.
Do more than fuck. Foreplay/sex/more foreplay/spend some time running your dick through the crevice of her vagina, making sure to bump your glans against her clit.
I particularly enjoy rubbing my dick through the lips of a vagina and bumping my head against the clit. Not only do I, but it seems that most women do too. Your dick is a wonderful clit stimulator. Don’t be shy to grab your dick and maneuver it around her clit with your hand, either.
Toys.
This may or may not seem taboo or impossible, but you might wanna try it. While having sex, reach down and see if you can insert a finger or two (or four?). Naturally, this will stretch the vaginal walls, especially the orifice of the vagina, and may give the illusion of that extra little bit of girth most women secretly want.
Masturbate together (don’t allow yourself to come if you can’t recover from it) before you have sex.
If you haven’t already, introduce her to and encourage her to practice kegeling.