There's an inherent paradox in motivating programmers. I think this Geek Hero Comic illustrates it perfectly:
It's a phenomenon I've noticed even in myself. Nothing motivates like having another programmer tell you they're rewriting your code because it sucks. Dave Thomas has talked about this for years in his classic Developing Expertise presentation, supported by the following quote:
Interestingly enough, a friend of mine (who is a quality control manager in a hospital) often makes identical statements in reference to doctors: Polite requests, coercion, etc. are useless at best and often detrimental. Peer pressure and competition are the key.
Don't try to race sheep,
Don't try to herd race horses
Yes, the use of the term sheep is mildly derogatory, but the general principle is sound: use motivational techniques that are appropriate to the level of developers you're working with. If you have neophyte developers, herd them with maxims, guidelines and static rules. If you have experienced developers, rules are less useful. Instead, encourage them to race: engage in a little friendly competition and show off how good they are to their peers.