Avoid letting any blade get wet as this will cause corrosion. Storing a knife in a leather sheath is harmful to the blade because moisture will easily collect around it. Wood handles can be cared for with oil of furniture polish. If your blade is composed of carbon spring steel, then use a baking soda and water solution to clean it. Stainless steel is much more susceptible to finger print than carbon and should therefore be cared for on a regular basis.
Finger prints are very acidic and can leave permanent stains if left uncleaned over time.
When choosing an oil WD-40 is always a good choice, but maybe not the best. WD-40, but without the odor. Another good product for folding knives is Remington's Dri-Lube. It applies as a very thin coat, but last a longer time than WD-40. A word of caution, Do Not ever apply more than a few drops of oil to any knife.