Hi Kim I have the same problem. All of our door hardware is solid brass, which looks great when polished, but then goes back to the tarnished look. You can tell if yours are solid brass/copper vs. plated steel by using a magnet. If the magnet doesn't stick, they are solid brass or copper.
I polish mine using an automotive rubbing compound to get most of the tarnish (oxidation) off, then use a laquer rubbing compound to do the final polishing. The difference is the size of the abrasive little particles in the compound. A polishing bonnet that chucks into a 3/8" drill is one way of doing the rubbing, as is a cotten wheel on a bench top grinder.
The trick is keeping them shiney. I've tried coating them with automotive polymer waxes (which work good for a while) and laquer sprays (which don't seem to last as long). It seems that body oils and mechanical rubbing remove most of the protectors we can apply ourselves. I understand there is a coating method that can be done commercially that is guarenteed for some period of time, but I've never researched this. Perhaps someone else with experience can post on this.