If you get a chance jump on enemies without landing and earn a combo bonus that increases the amount of coins you get from defeating them.
Also, your double jump is refreshed every time you stomp on an enemy.
Explore each level top to bottom. For instance, in the first world, each level will have a gem for you to find and you need to explore it.
From the second world onward, levels will start having crystals in them as well and/or keys that will open secrets for you later in the world.
Use your coins to upgrade dragons and increase their stomping power or maximum health.
Your dragons get experience when you beat a level with them.
Do the golden quests, short, mini levels, for bonuses.
There are two shrines, a normal one and a super one that can only be spent with crystals. These shrines will grant you potions, coins, and other goodies. Spend your crystals!