how bout this
every consecutive game you get without leaving you get 1 extra ip per match
ex. play 6 games in a row last game will have +5 bonus(first game doesn't get a bonus)
put a cap on how much it can increase
say 50 normal and 100 on special occasions, this will encourage people to play more often and not quit even when doing bad.
another option go to 50 then give the player the option to reset it back to +1 but the cap rises to 75, make repeatable up to 100 max bonus ip.
if someone then makes it to the 100 bonus give a summoner icon or ribbon to show that your devoted to playing or some other special reward