not using latex tubes, are you? they leak air and are essentially flat overnight. much lighter in some cses and a smoother ride with less rolling resistance, but leaky. mine go from 120 to 20 psi in about 15 hours.

most cheaper tubes are the butyl rubber type (geneally black in colour). these hold air much better, especially if the valve is properly inserted in the valve tube and ther's no grit around the seal.