As mentioned above, line quality is key.
You can try a shorter phone line from wall to modem
(a newer/shorter) an old phone cable may have some loss in line quality
If you have other phones attached to the same phone lines throughout your home - disconnect other phones when using your dial-up if possible
Optimize TCP settings, this small app can do that for you
(This will help but only get you maybe 1 or 2kb more, better than nothing)
SpeedGuide.net :: TCP Optimizer / Downloads
And lastly,
the fastest speed for a 56kb modem (in US) is
53.6kb by law