If you are talking about futaba-like servo motors the control is made by PWM pulses usually like this:

1.0ms = reverse (full speed)
1.5ms = stopped
2.0ms = forward (full speed)

so if you want to go forward at half speed your pulse should be around 1.75ms, if you want to go backward at very slow speed your pulse should be around 1.05ms or 1.1ms

search the forum, it has been discussed before.

orlando