FutureBlockHash.example.sol

mapping (address => uint256) gameWeiValues;
mapping (address => uint256) blockHashesToBeUsed;

function playGame() public {
    if (!blockHashesToBeUsed[msg.sender]) {
        // first run, determine block hash to be used                          
        blockHashesToBeUsed[msg.sender] = block.number + 2; // use 2 or more
        gameWeiValues[msg.sender] = msg.value;
        return;
    }

    uint256 randomNumber = uint256(blockhash(blockHashesToBeUsed[msg.sender]));

    blockHashesToBeUsed[msg.sender] = 0;
    gameWeiValues[msg.sender] = 0;

    if (randomNumber != 0 || randomNumber % 2 == 0) {
        uint256 winningAmount = gameWeiValues[msg.sender] * 2;
        msg.sender.transfer(winningAmount);    
    }
}