Focus on Dark Elixir

Once you reach Town Hall 7 and beyond, your limiting resource will always be Dark Elixir. Through normal play, you typically will max out all gold and elixir-based upgrades long before you acquire enough Dark Elixir to max out your heroes and Dark Barracks troop upgrades. As a result, you need to place a priority on Dark Elixir above all other resources, saving your spells and hero units for when you stand to gain a large chunk of Dark Elixir.