For me, finding good employees. It's an employee's market here, they can have their pick of the litter. I pay double the minimum wage, lots of raises, lots of opportunity for promotion, paid mileage and travel, bonuses up the yingyang, and I'm lucky if a candidate shows up for an interview!
Secondly, finding good clients. Reasonable clients who recognize that you get what you pay for. Referrals are your best bet here, so we focus on giving our favourite clients discounts when they refer us to a friend or family member. Also, we never haggle on price. It seems as though when you let a potential client even start to haggle with you, you lose their respect and never earn it back (or they're just the type to look down on you from the get-go due to your industry) We aren't the cheapest and we don't aspire to be. There is an over-abundance of cheap crappy cleaning "businesses" out there, I don't intend to add my name to that hat.
What would you say the most difficult or challenging aspect of window cleaning is? I too am looking to expand our services with window cleaning.