The best long term option is saving gems for entries to high tier challenges or Grand Challenges.
Only spend on Classic Challenges when a significant card in your deck improves.
Buying a Pass Royale early in the season gives best value through cosmetics and chests.
Avoid spending on drafting until you are very comfortable with random deck archetypes.
Use gems for occasional chest upgrades if you need a specific card or level increase.
Buying tower skins or emotes is fun but offers no gameplay advantage.
Prioritize progression and challenge entry to maximize gem use.