NOTE: Soon, my places will be replaced by a new map making tool, which can be previewed here https://mapsengine.google.com/map/
A few weeks after the rebranding of Google Maps as My Places, the interface lacked an explicit tool for exporting individual maps as KMZ. This feature was re-added recently. There are two links now, right under the title of the Place/Map, in the left sidebar, that allow you to generate and save a KML or visualize it in Google Earth. See yellow highlight in the picture below.