An Excerpt from https://cryptocointalk.com/topic/700-proof-of-stake-explained-rough-draft/ edited slightly for clarity:

"Eligibility is earned when your coins have sufficient "Coin Age". Coin age refers to the amount of time the coins have been inactive. Meaning they have not been transferred across the network or moved from the wallet. 30 days of coin age is required for your coins to be eligible to earn POS rewards. You do not need to have your client open or do anything to gain coin age. Just don't move them. You can have some coins that are eligible and others that are not [eligible] in your wallet."

     NOTE: With Coin Control, you can pick and choose which batch of coins you'd like to send from. The fewer confirmations (lower coin age), the better.

"For Example:

Say you receive 1 coin on Jan. 1st and another on Jan. 15th. You would be eligible to receive POS rewards on Jan 30th. ...Your POS reward, if you received one, would only be based on the 1 coin that is eligible.

How much will I receive for my POS Block?

The amount of the POS block you receive will be a % of your "staked" coins. Staked coins are the amount of coins you had eligible for POS rewards when you minted a POS block."