Turning down food politely

Scenario: You are a busy parent... you have just put time into preparing a meal or snack that has ended up being thrown on the floor, in your hair, and on your nerves. When is the best time to teach kids manners? When in their development are they ready to learn that this is not the best way to say that they don't want to eat the food or to even eat at all?
In a later post I will be talking about feeding cues, however in this post I wanted to mention the idea of teaching children to turn food down politely. This has really helped parents to be less frustrated at meal times.
At the end of 12 months a child has developed the ability to:

• Tests parental responses to his actions during feedings
• Tests parental responses to his behavior 
• Pays increasing attention to speech
• Responds to simple verbal requests
• Responds to “no”
• Uses simple gestures, such as shaking head for “no” 

By the end of 18 months:

• Says several single words (by 15 to 18 months)
• Uses simple phrases (by 18 to 24 months)
• Uses 2- to 4-word sentences
• Follows simple instructions

So you can start teaching them to say "please" and "no thank you".
When children are a little older, some parents use the "no thank you bite" so children learn early on that they are expected to try it in order to say no thank you to the rest of it.

How are other parents handling this? Please share what you have found works in teaching kids to turn food down politely or to communicate when they are through etc.

References to this posting:
CDC Learn the Signs Early  http://www.cdc.gov/ncbddd/actearly/index.html