There are these two stackexchange qna threads [0] [1] that I had bookmarked a while back. I came across another one [2] that mentions something called "bus factor"[3], which I found quite amusing, though this question is about freelancing. I think there is enough material in all these combined and the links they point to to sufficiently address your concerns.

[0] http://programmers.stackexchange.com/questions/125656/softwa...

[1] http://workplace.stackexchange.com/questions/9128/how-can-i-...

[2] http://freelancing.stackexchange.com/questions/126/how-can-w...

[3] https://en.wikipedia.org/wiki/Bus_factor