My advice is to maximize the price of your free entree. This is another no purchase necessary idea, but no price specified. So go ahead and try whatever you would like. For example, if you join Skeeter's Email Club, you get this free coupon to print out. I printed mine today and it doesn't expire till 4/4/10.
Why do restaurants do this thing? Because it is gets you in the door trying their food. If you like it, you will come back.