What's the best best way to preserve chrome wheels?

If it is a high quality chrome, it should be really simple. Keep them clean, using soap and water. (Windex is good for streak free quick cleanup. I had some Cragar SS's that I even used steel wool to clean, when the rust finally did come through in spots.) Keep them out of the road salt, at all cost.

If you are talking about the back side of the wheel or the part that you can see that is in by the brake rotor, you can use a good primer/pain to paint that black. It should keep the pitting to a minimum, there. (It IS very difficult to get paint to stick to chrome, as there is nothing for the paint to hold onto.)

Just "keep em clean", is all you can really do.
 
After washing them, make sure to dry them. The water sitting in the little crevasses and low spots with bits of sand and dirt still in it is what kills them. It creates the little pits that are often seen with mistreated chrome wheels…which eventually leads to rust and ruins the finish. :(