Do not attach any strings. Make it a straightforward offer with no fine print. People appreciate, and are more likely to respond to, this type of offer. Another tip for success is to minimize your printing costs. Direct mail costs money.
Brochures cost money.
Business cards and stationery cost money. Take some steps to keep your printing bills within reason. Shop for competitive quotes. There are usually a number of printers in a given area, so you should be able to find three or four to bid on your job. If you are having business cards, stationery and brochures done, do them all at once, advise the printer to quote each job both separately, and combined.