Ideally you want both; the commercial clients are great because they'll be cleaned what ever the weather throws at you, but, it's much more difficult to get hold of. Most guys start off with a domestic round, built up through door to door knocking, leaflet drops, etc and then gradually you'll start to pick up the odd commercial client, usually through being in the right place at the right time or word of mouth
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A letter through the letter boxes of pubs, clubs, shops, hotels, etc never hurts but it's a slow process and takes time. Also, if you spot a pub or shop, etc with dirty windows get in there quickly and introduce yourself as THE local window cleaner and ask if they'd like a quote. Once you're established in a street you'll usually pick up more and more work through people seeing you work.
What ver you do you will only get out of this business what you put in to it; sat at home waiting for the 'phone to ring will not make you a busy boy
Stick at it and you'll reap the rewards, unless of course you're planning to work in Plymouth in which case window cleaning is a wastre of time and you're better off looking for something else