When I had this a year ago it really got me down as no amount of stretching affected it. I then reinvestigated my running trainers, and whilst I had some good all purpose adidas galaxy, I got Nike Air Span Tri max, suitable for my foot-treading style, and the combination of this, but more importantly that the old pair's support had probably broken down since I'd clocked a fair few (maybe 500 miles) since the last pair, and that sorted it our immediately.
My tip is get the right shoes, and have a word with the sweatshop first as they have a foot monitor in most shops that tels you where all the pressure goes down on what are the best shoes for your feet.