Make it very easy to play again, and less easy to quit. For example, as soon as the game is over, display the user's current score and best score. Have them simply click the mouse (anywhere, not on a particular button) to play again. This will greatly increase the 'one more time' factor.
Of course, the most important thing is for the game to be fun. Nobody will play it again if it isn't fun. If they got a little adrenalin rush at the end of the last game, they'll be eager to give it another shot. Even calculation games can be fun.
High score lists with worldwide players only provide motivation to very skilled players. A beginner will simply be discouraged that he's ranked 8 millionth in the world. Early on, it's better to compare the user with his own previous efforts.
Also, it's nice to have intermediate goals. Provide the player with something to play for. It can be as simple as additional skins, powerups, or skills. Tantalize the user so he's sure he can get that next prize with 'just one more turn.'