Depending on the target market, one month free w/ a 12 month contract could be a very effective offer. However, as XtreemGreen pointed out, it didn't work in his market. (Although I'd be willing to bet it could work with some work on the flyer and possibly targeting!)
This points out the importance of TESTING.
The only one that knows if an offer will work or not is your target prospects and customers. It's foolish to assume you know what they want.
Not sure what kind of offer to make? Ask. Or, test out several different offers and the one that pulls best becomes your new control.