Once a dog has associated you, your voice and your hand signals with how he should behave, and knows what is expected of him and reacts correctly every time, the rewards can be reduced.
It may seem a little strange but scientific experiments have shown that animals will work better and harder if they are rewarded once every five times at random for the work they did rather than all of the time.
Once the sessions are easier then use the titbits for the new learning, or the best performances, and sometimes give extra special treats, games or praise for your dog.
A dog will work extra harder to win this recognition.
Make sure your dog understands what is required of him and has learnt the exercise well before reducing rewards as he will be confused and you will have to start again.