The FCC limits a 56K modem to 53.3 Kbps. The best thing you can do is run the latest drivers for your brand modem (Make sure they are from the manufacturer and not just the chipset) and as short and clean of a run to the connection box to the phone company. When running the cable to your PC, use a short cord to the wall jack. If you have the luxury of accesing your inside wiring, make sure the cable is away from any power lines, metal pipes and AC ducts. I get my best connections early in the morning (6:30 AM usually around 52.8 to 53.3Kbps) and the worst in the late afternoon to early evening. Connection speed varies greatly during the day.
Another thing I have tried with some success is using Cat 5 cable for the phone connection. Cat 5 cable is four twisted pairs of wire. The twisting cancels out the magnetic flux created when a signal is sent through the line and also helps cancel out any noise induced from external sources.
Hope this helps...