Training New Behaviors is About Making Patterns
The more times your dog performs a behavior successfully with a positive consequence (reward!) the more likely he becomes to do it again (whether it is a behavior we like or not)! Whatever he does now is likely what he will do in the future, so make sure those behavior patterns are desirable ones! Therefore, make it as easy as possible for him to perform the desired behavior initially so he is highly successful (and highly rewarded). Once it is easier for him because he has many past successes, change the criteria and very gradually make it harder while rewarding heavily.
Reward Appropriately
Keep rewards commensurate with the level of focus and difficulty of the task you are asking your dog to do -- this is as seen through the eyes of your dog. What is easy for one dog is difficult for another. What is easy one day may be hard the next if environment or circumstances are different. Dogs have off days, too. Recognize if something is harder and reward appropriately. Some rewards can be "too good" and become overly distracting or result in high arousal when we need calm and focus. Be looking for this and be ready to make changes.
Pay Attention to Arousal Level
This includes frustration, excitement, and fear. Frustration is often the ones that people miss. We can inadvertently raise arousal levels in our dogs by designing training scenarios that are too difficult for that particular dog or too difficult on that particular day. We may use the wrong reward (too exciting of a treat/toy), or train in an environment that is not appropriate for the dog. We have to start at whatever level is appropriate for that dog using whichever rewards and training environments set him up for success. A dog that is too highly aroused may not be in the ideal state of mind for effective training.
Pay attention to your arousal level, too. If you become angry or frustrated (it happens to all of us at times!), just step back and try something else or take a break! Don't poison your training sessions.
If Your Dog Keeps Failing then Change Something!
No one likes to play a game when they keep losing. If your dog keeps failing, find out why. If you aren't sure, break down what you are asking him to do and make it simpler. Reward handsomely! This will keep learning appropriate to where he is on that particular day, and will keep his interest in training with you. He should get it right at least 90% of the time or we are asking too much. Who wants to be told "Wrong! Guess again?" Don't hesitate to scrap the day's training plan if your dog is failing. We have to adapt what we do to the learner's needs.