Blind Coin Flip Strategy

You are blindfolded and sitting in front of a table with $N$ coins. Exactly $k$ of them are heads up -- you are told $k$, but you cannot see or feel which specific coins are heads. You are allowed to flip any coins you like (you can pick them up and flip them, but you still cannot determine their current state). Your goal: design a strategy that guarantees, after your move, both groups of coins (that you define) end up with the same number of heads -- no matter how the original $k$ heads are arranged. You know $N$ and $k$ in advance. Find such a strategy.