Don't put all you eggs in the one basket.

This could conjure up an amusing image of a wise peasant staggering around with dozens of baskets with a few eggs in each, but it is quite a sensible strategy in many walks of life. It is often applied to investments where you are advised to spread your risk so that one company failing doesn't wipe you out. It can also be applied to relationships - encouraging the idea of having a number of friends in case one lets you down. Having more than one job skill can also be useful when your principal employment is slack.