My best response was simply with a menu. I am currently in the mailshark yearly program and my sales are probably 50% higher than last year. I should say these results may not be typical as I am in an out of the ay location and have never done any kind of mailer before. But the program works.
As far as a postcard I would give away a high value low cost item. To me a give away is better than discounting as you are not lowering the value of your overall product or giving expectations of future discounts. Of course I have some discounted coupons and combo deals but my best overall performer has probably been a free stromboli with order over 25. The cost is right around $2.oo but for my menu the value is 9.50. I also stated a 9.50 value. Hope this helps