Well, first, many storefronts don't like their windows being cleaned on Saturdays, their typical busiest day, so you might have customers not pleased about that.
Beyond that, patience, perserverance, and looking out for things are key.
Go to a finer commercial neighbourhood, not a run down one. Go to the stores, ask to speak to the manager, offer a quote for window cleaning, and if he accepts clean their windows right away.
Building a route takes time and patience. It will not happen overnight. The fastest way to build one is to speak to as many people as possible, and get immune to hearing the word no. It is difficult for some to hear no often, but it has to be done. It's a numbers game and there are some that will say yes, because their window cleaner has abandonned them, or they are fed up with him, or the window cleaner moved away.
A good route may take weeks, or even years to build. It will start with a few stores, and as new stores move in, you will get them, and it will multiply. People will see you on the street cleaning windows and will ask you to give them a quote.
Getting new stores are the easiest way to build a route. Go after a new store, weeks before they open. If you see them put a new sign or awning up, or put a sign up asking for people to submit their resumes, that means the manager is in regularly, and persistently try to reach him. Establish a friendly rapport with him, make chit chat, give him your card. Later provide a quote, and chances are you will get the job. If you wait till opening day to get the job, you are toast. Some other window cleaner beat you.
You will also hear some customers getting their windows cleaned for ridiculous prices. don't let that deter you, or don't try to beat those prices. Generally if the windows are clean and the manager has a cleaner, move on. If you get a job by price, you will lose it for price too.
Also establish good pricing. It may seem easy to charge low prices, but you will regret it later. You will end up working for less than you think you are worth, and you will end up with the runt of jobs. Price at least $1/window for outside windows, and $1.50 for inside windows. Count the door as two windows, and always clean both sides.