It's worth noting that a traditional way of keeping motivation going is to allow a player to compete against a previous high score either a personal best or one set by another player.
This technique is a double-edged sword, however. On one hand, the player will want to continually get a higher score. At a certain point, however, the player's rate of improvement flattens out and previous scores are beaten only slightly, if at all.
When this occurs, the very thing that drove the player to try again getting a high score might now be a turn-off, because getting a higher score becomes difficult or impossible.