Focus on developing multiple TRAFFIC streams, not just multiple revenue streams.
Ditto what they said! Adsense can be replaced with ANYTHING if you have good real estate, in the path of high traffic, to put a billboard onto. (Start by keeping track of the advertisers in your adsense ads)
To answer your questions directly however... We keep a minimum of 6 months of our business operational expenses in the bank at ALL TIMES, and do not reach into ANY new expense until we have 6 months of savings for that expense alone, set aside to cover it.
For example... when we moved into our office, it was $650/month. We did not sign the lease until we had $3900 ON TOP OF the 6 months of operation expense, set aside.
All said.. we are just a small company, so saving for payroll does not factor into it.