At the end of a cobbing day, remember to make sure you haven't left any dips in the sides of the walls and poke holes in the tops of the walls to allow for drying and re-wetting. The holes serve another purpose too. They provide a key for the new cob to hold onto. When you continue cobbing, push the fresh cob down into the holes. When you've finished cobbing for the day, poke finger-sized holes with a stick in the top of the wall. Make them 3 or 4 inches apart, especially along the edges.
When you want to cob again, you can fill the holes with water if the old cob needs re-wetting. A hose with a spray nozzle is perfect for the re-wetting job. Keep some squirt bottles full of water around the cob site for re-wetting smaller areas as you cob. You will soon get the feel for how much to wet it so that you can work the new mix into the old.