Usually worn chain rings caused by a worn or stretched chain.

The smart way to keep the chainrings, and cassette from getting too worn is to keep the bike on a four chain rotation, from when everything is new. You swap chains every month or so (no more than 1000 miles), to keep everything at about the same level or chain life (stretch). This will just about quadruple the amount of life you can get out of your rings/cogs. What really truly causes ring/cog wear is not unlubricated chains or even mileage, but a stretched chain.