A major part of training your dog is rewarding good behavior with a treat.  This sends a positive message to your dog that a certain behavior was the right thing to do and that he should want to do it again in the future.  Always make sure you have a healthy treat ready to give to your dog when training.  Don't over feed your dog treats. Treats should only be around the size of a pea. Treats should only be given to your dog when he does something you want to reinforce. Dogs can eat many healthy treats.  Try offering bits of carrot, sweet potato, or a slice of apple. Dogs learn new tricks best when you train him in a quiet, distraction free environment.  If there are too many other things going on, your dog may not give you his full attention.  Always make sure that the place you are training your dog in will allow him to fully focus.  For example, avoid a busy public park.  Try to find a quiet space that your dog can focus in. Familiar environments are better than new ones. Just like humans, your dog can get stressed out if he feels he isn't getting things right or is doing the same thing too much.  Whenever you are training your dog, try to avoid any prolonged sessions or pushing your dog to learn something too quickly.  If your dog seems to be having trouble learning a new trick, take a break and come back to it later.  Taking a break can be more effective than pushing through. If you think your dog has lost interest, stop for the day.

Summary:
Get some treats ready. Keep the space calm. Don't force anything.