If I think I can finish it in a day or two, rent from redbox.
If I think I can finish it in less than a week, rent from Family Video.
Otherwise, digital. I never pre-order.
Within the digital space, I have been known to buy codes from outside the xbox digital store. Amazon occasionally has the digital codes cheaper than xbox.com, and then there's always cdkeys or boxeddeal, etc. Otherwise, I troll cheapassgamer.com for posts where to find deals, and the trading forum is excellent for buying codes from people for really cheap. Even if you can't find the game you want, people always sell gift cards for a chunk off. Finally, I'm obsessed with bing rewards -- I know it's not everyone's favorite, but you can easily get enough to get $10-20 in free MS money each month.