There are no hard rules and what worked for a famous breeder may not work at all for you; it's a mix of pedigree analysis, timing, expert eye, common sense, and a little bit of serendipity.
With a little bit of serendipity thrown in for good measure — Nancy is mistaken for Jessica (the would - be date) in part because she happens to be holding the same book Jack has been reading — our adventure plods onward through the streets of downtown London and into pubs.