Try mounting your tires with the lable right over the valve stem-- When you find a slow leak in the tube you can narrow down your hunt for the culprit by laying the tube on your tire, with the valve stem on the tire's lable, and checking out the region of the tire near the tube leak. I usually end up checking two areas because I can never remember how the tube came out.

Also an old nylon stocking (if you can find one) used to wipe the inside of the tire will snag on some of the more obvious penetrating objects. Some of the fine pieces of wire like those from steel belted car tires will not show up until there is pressure in the tire. That is why it is improtant to really narrow down the search area and look very closely. Pertuding spoke heads can also cause problems.