I have a special promotion condition and not sure Magento can do it or need other third party plugins!
anyway here is my condition: If a customer put promo-code xxxxx and they will get xx% discount on their next order(only get this discount on next order not when promo-code puts).
Please have a look at this module - Special Promotions Pro. It will let you create a discount rule for those clients that have already purchased something in your store. Please let me know if you have any questions.