What's the speed rating of your modem? Is it a 56K modem? Before 56K, there were 33.6K modems and, before that, 28.8K modems. If you have a 33.6K modem and you consistenly see 33.6K shown as the actual speed, then you're getting as much speed as possible with that modem. You would need to get a 56K modem to get better speed.
If, on the other hand, you have a 56K modem, then the line quality GX1_MAN mentioned is probably limiting your actual speed.