Photo 1:  Make this specialized clamping caul using a dado head or router bit to plow out the material between the teeth. Cut the caul to length after the teeth are cut.

Photo 2:  Since the caul puts pressure on the tails only, it’s easy to seat them completely into the sockets.

The best way to make a dovetail is to allow the end grain of the sockets to project slightly past the face grain of the tails, then pare the end grain flush after the glue is dry. But getting the joint to close properly can be tough. The head of the clamp hits the end grain of the socket board before putting pressure on the face of the tail board, so the tails don't seat all the way into the sockets. Moving the head of the clamp to avoid the end grain can bow the tail board. A specialized caul is the answer, Photo 1.

Each tooth on the caul corresponds to a tail on the joint. Since you've taken away the material between the teeth, you're not putting any pressure on the end grain of the sockets, Photo 2. This allows you to push the tails all the way into the sockets.

This technique, using a different caul, also works great for finger joints.