Surface cracked tires are no longer the plague of RV owners and Classic cars. Tires today can last to the 80-100 thousandth mile. When exposed to sunlight and the elements they can develop those small cracks often referred to as weather checking, ozone cracking, or just plain sidewall cracking. Slight surface cracking can be just cosmetic but keep in mind deep cuts or fissures from excessive deflection, poor maintenance, or defects can be serious so check with a tire shop if you are unsure.
There are steps you can take to minimize cracking. Tires have oils in the rubber that like to be flexed so they can move around and stay dispersed. So try and drive those tires from time to time. Give the oils a fighting chance and cover the tires if they are going to be stored outside for an extended period of time. If you have separate snow tires, try and find a spot in the house where the temperature and humidity don't fluctuate. Some people go as far as bagging each tire in the off-season to help keep those oils in their own little micro environment .
Keeping tires clean from brake dust and road grime will help as well. However, if you are going to clean and dress your tires one of the best things to do is stay away from silicone based tire shines. Silicone draws out the tire's precious oils which will contribute to cracking. Try and find a water-based dressing and rubber specific cleaner. There are several good detailing companies like Griots that specialize in high quality detail products that will keep your tires looking fresh.
Check out some of Griots Tire and wheel detailing products for wheels and tires.