As an added thought, and noone jump my xxxx here

You could run these competitions more often, but make them code based.
ie. add a coupon code like "OrderDraw" that doesn't give a discount (or perhaps just a 5% one) - or it can give the 10% discount that everyone loves, but make this a requirement to win the order # prize. Then you can have one every 100 orders. Most would be unclaimed, as people would be using other discount codes, or just not know about it, but for those that 'like to gamble' or just want to be suprised, and use that code every order, there might be a winner 1/10 times or something... anyhows, just an idea, I should have posted it last week on the competition ideas
thread lol.