Using more than one voltage regulator in series
To add to the other answers, here is some general information about getting a low noise power supply from a car:


The voltage from your car will normally be between 12V (engine off) and 14.5V (engine on) but may have drop-outs, glitches and spikes up to about 30V. So you are correct to think about smoothing it for precision use.
The simplest starting point is a DC-DC converter, such as one described in the other answers. On its own, it'll give you the set voltage with probably 100mV or ripple, depending on how much you paid for it.
If you need a cleaner power supply, you can pay for a better DC-DC converter, or you can add a filtering stage. A low-ESR ceramic capacitor connected across the output is a good start, and a LC pi filter will do better, but be careful to select low resistance parts to make the filter.
If that's still not good enough, you can set your DC-DC converter to 14V and use a linear regulator to drop the voltage to 12V. That will usually produce much lower noise on the supply, but make sure the linear regulator has a good PSRR at the switching frequency of the regulator.
For absolute best results, use a DC-DC converter, then an LC filter, then a linear regulator, and finally a large bypass capacitor near each IC you are powering.  Put the DC-DC converter in a metal box, the regulator in another box and your actual circuit in a third box, so they are all shielded from each other, and use shielded cable to connect the 2nd and 3rd box.


There are very few applications where you'll need to go beyond a decent DC-DC converter and maybe a capacitor. Taking all of the steps above would only be necessary if you were doing something like powering an ultra-low-noise instrumentation amplifier to make sub-nV measurements.