I find the fastest way to sail on the flat off or run, is to be comfortable and catching waves. Lots of books and people tell you that you need to get the boom out square and sailing by the lee. You find this is fast but very uncomfortable, and easy to capsize.
If instead, you pull the boom in so that you are comfortable, and the boat does not want to tip in backwards, you will find it a much more comfortable ride. You can then concentrate on catching waves, once on the wave you can ease the mainsheet out, and steer down the wave, and by the lee. You will find the boat will become stable, once at the bottom of the wave you can pull the mainsheet back in and steer up the wave, and become comfortable again. You will find if you do this quick enough, you will accelerate and probably stay on the wave.