The Molecule "Refresh" works great for getting rid of stink, we sell it online and have it at the races. Molecule also sells a wash product for suits that will also work on removable helmet liners.
The thing to NOT do is zip it into a gear bag as soon as you take it off, it needs to air dry first. Using an air hose to get most of the dirt out is a quick and easy way to keep the dirt from getting ground in tot he liner. I use an air hose on my car and moto helmets before they get put away between use.
Air hose, not a water hose. I cant see ever needing a pressure washer to clean a helmet, especially the inside.
If you race in mud,keep it waxed or hit it with pledge on race day and the mud will come off easier.