Looking into it a bit, it appears that this particular fun fact was first thought of (or at least put to paper) by von Neumann in "Various techniques used in connection with random digits" in 1951. Yuval Peres proves in "Iterating von Neumann's procedure for extracting random bits" that if N goes to infinity it approaches the entropy limit.

So for particular small choices of N there might be more clever schemes, but for large N you can't do a whole lot better.