The most efficient way to get more Dust is to complete daily quests for Gold, enter the Arena when you have enough time to do so, and compete in the ranked ladder when given the opportunity. You won’t gain a lot of Dust this way, but the steady trickle will accumulate over time. Also, keep an eye out for a new Tavern Brawl. Your first win in a Tavern Brawl will reward you with a card pack, which you can open to dust the cards for value.