I find this the easiest way to train your monsters without them getting tired. I put in various ways for the level of the gadget you decide to use:

LEVEL 2
Week 1: Level 2
Week 2: Level 2
Week 3: Level 1
Week 4: Rest

LEVEL 3
Week 1: Level 3
Week 2: Level 1
Week 3: Level 1
Week 4: Rest

LEVEL 4
Week 1: Level 4
Week 2: Rest
Week 3: Level 4
Week 4: Rest

Here's an example of how to do this, I'll use a Level 3 schedule

Speed Training
Week 1: Pitching Mac
Week 2: High Jump
Week 3: High Jump
Week 4: Rest

This work all the time and if the monster has a tiny slither of fatigue in him just use an Oily Oil on him. You can buy them in Visley.