I just mine altcoins that seem interessting till i have 10-100 (depends on difficulty etc.).
In this way it won't really cost me anything and i still have the profit if the coin might shoot up in value somewhere this year.
Besides that it's just fun.
How do you decide which altcoins to mine? I started with COYE, you can guess what happened...
Are there any signs i can look for, that indicate a promising altcoin, or is it just luck?