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?
If you want to make real profit it's just luck.
As i said, i just try to get as much different alt coins as possible so if any do make it i will gain something.
Mostly i just take the coins that already have a nice market cap, so those coins already have a solid base.
For the rest it's just random, but i do mine a lot of doge. Currently Doge is the most profitable coin to mine (if you trade it to BTC and then sell it).
Just check out the multipools and see what they are mining