Jesus,

Write the firmware so that a single switch depression for X milliseconds causes an increment of a variable representing the minimum allowable increase/decrease of servo speed setpoint. If a switch is depressed for longer than X milliseconds, allow the increment / decrement value of the variable to increase. After the switch is released the increment / decrement value returns to its minimum value so the next switch press can control the speed set at the minimum resolution.

As Oleg suggested, use two buttons - one for increase and one for decrease. I suggest adding a third to give an emergency shutdown.

Must the servo acceleration and decceleration be limited for any reason? If so, let that limit value be represented the maximum allowable increment / decrement value of the speed setpoint variable controlled by the switches.

Have Fun,

Bruce