We set up the expectation at very beginning - don't have to get A+ but no B. We don't check their grades except at the quarterly report. If we saw a B+ or an A-, we would set up an appointment with the teacher first to understand the reason. After that, we work with our children to develop a plan to ensure an A as the final grade. When they get that A after some catching up, we give them a lot of positive encouargements.
The only B our son had were partially our fault. He took an AP that was offered to all grades with a very tough teacher who gave very few As. We should have known that and don't let him pick that class. He was one of the few 10th graders among most 11th and 12th grade students. So, dispite his efforts, he got a B. The GC said that may be the high grade for 10th graders.
Bottomline - make your child want to get A themself. Push will not work. Too much push will have very negative effects, believe me.