What's the best way to give existing customers extra value and discounts without giving away the store?

[willstauff]

Anyone know the best way to structure discounts for existing customers?
What usually works best without giving away the store. When things get
slower, do you provide discounts? What’s the best way to do it?

[trcapromo]

Faster completion of work? Two gigs for the price of one? Those seem like simple ideas.