Since it is an interior surface I would use a water-based sealer purchased from a concrete supply house (they should be able to recommend a good brand). Apply with 3/8 roller after floor has been cleaned and dried. Once that is dry a mop on wax can be used as a sacrifical coat. 2-3 coats applied with a micro-fiber mop. For more info http://www.tomaroconstruction.com/care---maintenance-for-interior-floors.html

A quick review of your cleaning products/habits would help. Many flooring cleaners will dull floors prematurely. I'm afraid I'm a little weak in the "stained/stamped" concrete flooring so I'm not sure if fading is considered routine or not.

Your best source, before doing anything exciting (like buying product) is to contact the concrete finisher and find out if they carry product specific to your floor. While you have them on the line, you can then ask if "washed out" colour is to be expected with your form of finish/install.

These are some excellent questions that can be answered quickly and appropriately by the installer/supplier.

A ph neutral cleaner will work. Wring rag or mop out, scrub and either towel dry or air dry..