Gold mainly pays for defenses, Elixir funds many offensive buildings and troops, and Dark Elixir is used for heroes and Dark Elixir units. Your priorities should therefore reflect your available builders, laboratory schedule and play style.
Avoid idle upgrade time
Start a long upgrade before a period away from the game and keep a shorter option ready for when a builder becomes available. Keep the Laboratory researching whenever possible because a builder cannot recover lost research time.
Choose profitable targets
A worthwhile attack depends on more than the displayed loot. Consider your army cost, training time and the likelihood of keeping your trophies when league position matters to your goal.