If you have a facebook page for your business, take advantage of it for the sale. You should list when the sale starts, your discounts, and videos showing the merchandise you carry. Do not take pictures of empty shelves later in the sale and post them on your Facebook page. You want to give the appearance of having a good selection, even when you are picked over.
SMS ( Text Messaging Marketing) is the way to let customers know about future markdowns. I used to have email marketing too, but I was getting 10+ SMS subscribers for each email subscriber. Do not send more than two messages weekly.
Cheapesttexting.com has a great pricing for a monthly plan. You have limited characters per message, but include business name, hours and your discount % off.