Your doors have knobs and handles that are usually made of some kind of metal. These knobs get used a lot by your hands which means they often times get dirty and perhaps even have some bacteria and germs on them. Get in the habit of periodically wiping down the knobs and handles and perhaps even finding the appropriate product to give them a little polish.
If you happen to have brass door knobs, these are especially elegant but often times get the dirtiest due to the oils that build up over time from people’s hands. The shiny brass color starts to diminish and get dull over time.
A way of cleaning these is to use a lacquer thinner and a fine steel wool brush to get off all those oils from the door handles. Make sure you wear gloves when doing this, and be sure not to let the lacquer touch the door itself, as it will damage the door’s finish.
After, find a brass polishing product and apply to the door know, again using gloves and avoiding contact with the door. Here is a great video from Expert Real Estate Tips that shows you how to get it done.