Short answer is: save potions
Use less heroes in quests/pve map encounters.
Avoid loosing energy in general and play passively which means to do quests first then war.
Try not to get routed or pvp often...
Look at your next castle level and unlocked buildings. Then focus siege only for those rss needed to upgrade your next batch of troops.
Main sources are potions on map, quests, patron, witch shop, void, event boxes, sometimes ingame shop and recently random pve map encounters also drop potion rewards. Lots of stamina will also increase your hero energy limit.