Coins are earned in a similar way to cred, in that the amount you earn on each song depends on the overall difficulty, and your star rating. Every new song you play will earn you double the amount of coins you earn, which you can use this to your advantage and try to get five stars on your first try (with at least one powerup) to obtain more coins than a regular play would earn you.
This will probably be best to obtain alongside with "Precious Medal" so you don't use your coins too quickly.
Also, if you have a facebook profile, you can connect to Rock Band World to compete in challenges which can earn you more coins. However, some of these challenges will require DLC.
Note: You cannot earn coins if you're not connected to Rock Band Central.