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?
It was kinda obvious kanye was going to sue the devs and halt the project. All these copycat coins brings nothing new to the market so they won't survive the long run. Look for innovation and unique attributes in new coins. I try to mine the coins as they are launched and sell them if it gets listed.