You just need to be in the right place at the right time. Commercial is a hard slog and there is little if any loyalty. Its mainly about price. I have a large commercial client base and it took me 10 years + to build. Everybody thinks commercial is the best bit in reality it is hard work, not only cleaning them but keeping them.
Get a good solid domestic base to work from and then ask your customers, tell them you are looking to do more commercial and ask them if they have a window cleaner at work, when you have proved to them you're not the usual here today gone tomorrow w/c they will have confidence in recommending you, especially if you've been doing it for 12 months or more, been reliable, seen the winter through and do a good job.
best wishes, Trev