1. Treat interior and exterior windows the same.
The exterior and interior windows have different degrees of grime, window cleaning supply manufacturers make different scrubber sleeves for them.

2. Mixing up a solution with too much foam.
It is better to use less soap and add a wetting agent of you need more glide.

3. Taking too much time.
that people don't like to wait around for an eternity to have their windows cleaned. After all, your service is supposed to be a convenience.

4. Wasting gas and time driving around giving free estimates. Don't do it.
You can do this thorough your website or over the phone when potential customers call you. Advertising that you do free estimates, and then driving all over town just to tell them how much it may cost will drain you of time and money.