I agree, encouragement and more importantly praise is key. My mom expected nothing less than A's. I was a gifted and talented labeled child and in honor/AP all through school so why shouldn't I get perfect grades? If I brought home less I got nagged and yelled at. If I brought home straight A's I didn't even get a pat on the back. Needless to say it was a bit wearing and if I hadn't wanted to get into a good college I would have slacked off since I couldn't win either way.
Alot of people use money or things to reward good grades, but I think family time works better. If the child gets good grades promise a picnic or a hiking trip or something they would enjoy, with everyone from the family. Even a pizza night to congratulate them would be great with everyone talking about how great they did.