This is the key, I think! Your child has talent and brains, but if they don't want to use those talents, they may not do as well as they could.
Positive reinforcement is always good. We have found sports to be very helpful. My son is very small and for him, just being on the swim team is a success. He's never going to be in the Olympics, LOL! Pointing out how he dropped time at the meet, or noting in a good way how another kid has swum fast after he has worked hard in practice.
Gradually, my son has become motivated in other areas. He has really embraced a reading contest at school, trying to read a certain number of books in a specified amount of time. As he has slowly achieved that, we make a big deal about it, going out to dinner or something like that.