If the first frequent flyer rule is to concentrate on one program, then the second rule is to have a diversified mileage portfolio. For example, I maintain Plat and have most miles with AA, but I try to keep at least 80K each in UA, NW, DL, and US. That said, only once did AA not have award seats for when I wanted. Fortunately, as it was a family emergency, I was able to use UA, and for fewer miles!