I made the mistake of taking on big jobs which led to even bigger jobs and so on (commercially). Trouble was, when I lost one housing association contract, I lost several other jobs all related but in other locations.
Now I only do jobs which take no longer than 5 hours and never keep all my eggs in one basket.
I vary my work by locations, types of buildings, commercially, industrial and domestic. I only have a small percentage of regular holiday accomodation property and second homes which I set a limit on so I don't leave myself short of regular work out of the holiday season, which ensures my mortgage and all my other bills get paid promptly.
Yes it's very nice to charge double or even quadruple the price on certain jobs and new cleans but I look after regular customers as my first priority.