1. If you book when there are still many empty seats, choose a window/aisle seat near the back of the plane. Those middle seats are usually the last to be filled.
2. If you're traveling in a group of 2 and there are 3 seats per section, choose the window and aisle of an empty section (preferably near the back of the plane). Chances are good that if there are any empty seats left on the plane, it could be your middle seat. If someone does happen to book that middle seat, I think 99.99% of the time that person would be happy to change to the aisle/window so you can sit next to your partner.