Comfortable mouse tracking speed not available in system preferences
You can adjust the tracking speed on the command line through the Terminal:

defaults write -g com.apple.mouse.scaling x.x


Where x.x is the tracking speed, 5 being the high and 1 being low, e.g.:

defaults write -g com.apple.mouse.scaling 3.5


To reset it to defaults, use:

defaults delete -g com.apple.mouse.scaling


And it should reset back to the default tracking speed. Play around with the increments and see if that's fine-grained enough to find the tracking speed you want.