How does PoS work?PoS can only be done by an unlocked wallet. In the client it is called 'Staked Mining'. As soon as you unlock your wallet (there is a difference between encrypted & locked) the stake miner will start. You will start to see coins being moved from your balance to your 'Stake' balance. Coins will not be lost. After a while you will start receiving transactions with your earned interest.
How to unlock:Go to your Debug Window and then open the Console. Type in:
walletpassphrase -yourpassword- -secondstokeepwalletopen- true
An example would be:
walletpassphrase Ex4mpl3P4ssW0rd 3600 true
In this way you'll unlock your wallet for one hour.
How to lock:Go to your Debug Window and then open the Console. Type in:
walletlock
Now your wallet is locked and you'll stop PoS mining.
Is my stake balance spendable? - Not sure yet, has to be confirmed by either dev or community
This is the information I have managed to gather so far, If you have any other questions, please leave them here so I can try to answer them!
Also if people would like to reserve a certain amount of coins that they would not like to be staked, try this.

reservebalance true AMOUNT
example: reservebalance true 500000