Your energy bar normally refills to a maximum of 100 Energy (or 150 Energy, if you're a subscriber / donator).
For wars / chains, you want to have maximum energy at the start, therefore stacking gives you a head start.
1. Empty your energy bar first. Make sure it's 0.
2. Use a Xanax to get 250 energy.
3. Wait until the Drug cooldown is over, usually between 6-8 hours.
4. Take another Xanax. Keep repeating this until your energy reaches 1000.
5. Your energy cannot go more than 1000. So, do NOT use a Xanax if you have more than 750 Energy.