I use grease on my battery terminals - after cleaning them. I use the Dow silicone #111 (I've got a lifetime supply!) It doesn't affect conductivity - it squeezes out as the contacts come metal to metal - but it does keep moisture out and helps prevent corrosion. For the post type connections you can also get some felt pads that go at the base of the post and help prevent corrosion. Don't use them currently as batteries are stud and nut connection.
You can check for bad connections using a digital voltmeter on low range. Put one probe on the battery terminal and the other probe on the connector on the end of the cable. With some current flowing, you shouldn't see more than a few millivolts across the connection interface.