Originally Posted by spcialzdspksman

eeGet a new tube. I had the same problem once. There was a leak, so I patched it, but overnight, all the air leaked out. There's probably a crazy small leak somewhere, but I don't think it'll worth the trouble to find it and patch it again.
Also if you're using tire levers to mount them, be really careful not to push in too hard or it will puncture the tube.
You can either pump a little air into the tube (to give it shape) before mounting, or just use your hands to mount them (says LBS guy, although it's a pain to the fingers and stomach).

edit: one more thing, make sure the tube is properly seated along the entire rim, (meaning none of it is pinched between the tire and rim) before pumping air into the tube. To do this, work around the rim and wiggle the tire in a forward- backward motion perpendicular to the rim.
Hope this helps!