Each out-of-stock product page shows a "Notify when the item is back in stock" link below the "BUY" button. There is also an "Add to Wish List" link on the right side. This will come in handy later!
Close-up of sign-up link:

When you click this link, the product will be added to your Back in Stock Notification List.  This section will now display links to "Stop notifying me on availability" and "Manage notification list".

Click "Manage notification list" to see all of the products you have signed up for.  On this screen, you'll be able to view the products on your list or remove/stop notifications for specific products. Once the product becomes available, you'll receive an automatic notification and the product will be removed from this back in stock list. This is where "Add to Wish List" will benefit you.

The benefit of adding to Wish List, in addition to the Back in Stock Notifications, is that all products are added/removed manually. You can add any products that you want to revisit to this list, they do not have to be out of stock. All products have an "Add to Wish List" link in the right-hand column (see first image on this page). To view your Wish List, go to the login icon at the top of your screen and click Wish List.
From the Wish List screen, you can Buy or revisit products you've added to your list.