My first thought would be a speed controller, something along these lines:
The speed regulator uses PWM technology, which has the characteristics of high conversion efficiency, wide speed regulation range and stable performance. -DC 9V~60V DC Speed Regulator. Speed principle:Current regulation.
www.ebay.co.uk
If you replace the pot with a capacitor and resistor, so each time power is switched on the speed input starts at zero and ramps up, that should give you the soft-start effect. Add a reverse diode across the resistor to quickly reset it when power is switched off.
Or, if the motor load is low, just a power resistor to limit the initial current and short that with a relay after eg. half a second, using a capacitor across the relay coil and resistor feeding it to delay the relay switching on.
Again, a reverse diode across the resistor will give a fast reset.