As root, open the file /etc/X11/xorg.conf -- then look for the section related to your touchpad (could be 'Mouse', 'Synaptics Mouse', etc.).
In that section, look for the values 'MinSpeed', 'MaxSpeed', and 'AccelFactor'. If they're not there, you'll have to add the lines below.
These values are just an example, so feel free to change them to suit your needs -- this is just how I like it.
Option "MinSpeed" "0.10"
Option "MaxSpeed" "1.50"
Option "AccelFactor" "0.0300"
save the file. You'll have to restart X, I think.