It is important to keep the sessions short and fun. For young dogs its best to keep it too around 5 minutes, as they get older and their attention span increases you can increase the time of the sessions.
As the dog learns that the sitting after hearing the command sit, means he gets a treat, you will notice that you no longer have to trick the dog. However, at this point it is important that you keep giving the treat. Once your dog masters one trick its time to move on to something else.