We have a natural desire for thing that are (perceived as) exclusive or belonging to a selected group.
Many websites who launch new features have a "Private Beta" which creates interest for the new features. But you can also apply this to online shopping by creating 'Private sales' shops or categories: only users who are invited can create an account and view the products and promotional actions. You can use this for your complete shop or a certain category of products. You could give your most valuable customers 'early access' to new products of give them exclusive access to cheap end-of-life products.
There is a screencast from Magento explaining the possibilities of Private Sales with Magento.