In nature, dogs are almost never away from their pack. It is our job to help make this unnatural situation less stressful.
This is what Cesar Millan recommends:
Before you leave, go for a walk. Start the day by providing vigorous exercise. Then reward your dog's calm-submissive energy with food and water. Some dogs may need to rest before eating, but all dogs can benefit from hydration. The idea is to leave your dog in quiet, resting mode while you are away.
No touch, no talk, no eye contact. Don't make a big deal when you leave for the day or when you return. This way, you are communicating to your dog that the time apart is no big deal. It's just business as usual! Depending on the severity of the case, you may need to practice the rule for five minutes or up to an hour before you leave and when you get back.
Say goodbye long before you leave. Having trouble practicing "no touch, no talk, no eye contact"? Take a moment to share affection and tell your dog that you will miss him way before you actually leave. Keep in mind that this display is for you - not your dog! Your dog won't have his feelings hurt if you didn't say goodbye.
Stay calm and assertive! When you are ready to go to work, leave those guilty, nervous, and concerned feelings behind. Instead, let your dog know that everything is going to be okay by projecting the confident energy of a pack leader.
Start out small. Leave your dog alone for five minutes. Then, extend the time to twenty minutes; then an hour. Continue to increase the time you spend away until you can leave for a full eight hours with no problem!