Promotions: What are the best practices?

So, I'm constantly looking at my promotions section thinking, "Is there anything I can do to improve my turnover and retention rates using this system?" I've essentially got all of Autopilot on and set to Smart Discount, and ran a 50% off discount code for returning Cancelled members... anyone got any advice/recommendations on better uses of the systems or better ideas? I'm always wondering if Smart Discount might be worse than a set percentage...