I think it's better to plan for the cheat; that way, it'll be easier to get back on plan. It won't be a likely to turn into a three-week carb binge!

That said, I don't think cheating is really worth it, but if you want to, plan ahead. Decide now what you're going to have, and don't overindulge in portion sizes. I'd recommend choosing one item. Don't go for the baked potato, breaded seafood and giant slice of cake. Just get one of those things, if you can help it. It can still feel indulgent, but you won't be going whole-hog.