You don't say where you live and so this is based on generalities. The speed of your modem is a function of the standard of telephone line that you are using. The bit that goes from your property to the exchange (Usually BT in the UK) is the bit that counts the most but is the bit that you can't do much about. However listen to your line when there is no speech and if you can hear noise (bangs, crackles, etc.) tell BT you have a noisy line and they MAY be able to fix it which will make your data go faster. (Don't mention that bit to them because they don't have to fx it - just say that it is noisy.)
Next use the master socket only and check that the line connections are secure.
Finally use a good quality cable between your modem and the wall socket and make sure that it is not routed near any power cables. (This usually has least effect.)
Terry
September 2006