Sell to your old customers.
When I first heard this very simple idea I was amazed. For some reason we game developers think of how many copies we can sell – we miss to notice that selling once to 900 customers is the same as selling 3 times to 300 customers. And believe me: finding 900 new customers is (about) 3 times more difficult than finding 300 new customers.
I believe this is a very important subject when thinking where the money comes. I believe creating subscription based (monthly/quarterly/yearly payment) games are something that indie game developers could use. Others use expansion packs. Some developers use different skins to sell the game again.
Selling to an old customer (again and again) is good because:
1) It benefits both sides: the player wants to get more content and is willing to pay to get a better game – and you developer can continue developing the game with the funds received.
2) The customer already trusts you: it’s easier him to make the purchase decision – again.
3) When you have collected the emails of your customers (and they have accepted for you to email them) it’s easy for you to reach your old customers. It costs less money than advertising.
Remember: think about the life time value of your players.