Ask HN: Do you have any rules of thumb for when to hire?

More specifically I am asking about the timing for specific positions. Obviously having the cash flows to hire is a key but what other metrics have you found valuable to look at? Has anyone found any useful heuristics or books for, as an example, deciding between another developer vs another sales person? The majority of the material I've read on the subject has been geared more for larger organizations and not a small series-blah startup.