Well, it's basically built-in.
- a barbarian can enter and end rage as a free action.
- After rage, barbarian is fatigued.
- Barbarian cannot enter rage while fatigued or exhausted.
Then, they get all these rage powers, some of which are useable only once per rage.
Say, Strength Surge, Surprise Accuracy, Spell Sunder.
So, step 1. be a barbarian
step 2. become immune to fatigue
step 3. profit.