By far the best way to learn arithmetic facts is to ‘naturally’ use them in service of solving more interesting or relevant problems. The same amount of time spent doing nontrivial word problems, pattern-discovery projects, playing a game or solving a puzzle involving embedded arithmetic, or just talking about numbers in a group, is vastly more useful both to arithmetic ability per se and to mathematical fluency in general than narrow practice drills, which are not only a huge motivation killer for most people, but also just suck at aiding retention.