A good idea is to put a large (the larger the better) capacitor on the incoming power. Just put it between + and - (make sure you get the polarity right, large capacitors usually require that).
It will have several good effects:
* The capacitor acts like a reservoir, which can store a little power, enough to smooth out small dips.
* The capacitor acts like a filter. When starting and stopping the engine, you can get power spikes. These can potentially damage electronics. However, these are very short, in other words, high frequence. High frequence signals pass straight through a capacitor, so they are harmlessly led back to the battery, without reaching the device.
* As it filters the power, it also reduces hum that might occur from interference from other systems.
A very cheap insurance against problems. Costs next to nothing, and, if you are cheap, you can probably salvage one from an old computer power supply.