Three researchers in Europe offer this answer: subtly.
More specifically, their research finds that offering students extrinsic rewards for their retrieval practice reduced its effectiveness.
Students offered rewards made more mistakes when they first tried to recall information, and–even taking those initial errors into account–remembered less than their fellow students who had received no enticement to practice.
In this study, the extrinsic rewards were cash payments: students received a euro for every correct answer. In schools, we rarely pay students money to get correct answers. However, we quite often pay them with grades.
This study suggests that retrieval practice should–as much as possible–come in the form of very-low-stakes or no-stakes retrieval.