Some dogs will lay down and relax at the smallest excuse. Others can't seem to settle on their own no matter how tired they are and behave like an over-tired toddler. If your dog falls into the second category, don't worry, there is hope!
My dog is the same and struggled to settle down and rest on his own. Karen Overall's Relaxation Protocol was recommended to me by another trainer and has been a sanity saver for me and my dog. This protocol can also be an great tool for working with reactive dogs.
The relaxation protocol uses the principles of classical conditioning to reinforce calm behavior in a wide range of conditions. The protocol starts easy with your dog sitting or laying on a mat while you challenge them with small distractions like taking a step away and back or quietly counting to 5. Later on your dog will learn to stay calm and in place as you walk out of sight or go to the door and pretend to talk to a guest. There a total of 15 different task sets you can work through with your dog. Don't worry your dog (and you) will benefit even if you choose not to work through all 15.
I have put together a set of tools to help you use the relaxation protocol with your dog.