You do have a Pesticide Applicator's License, don't you? The 1972 Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA) mandated that each state certify and regulate the commercial users of pesticides. The procedures and requirements for obtaining the certification vary from state to state, but there are stiff penalties in store for you if you or your employees apply regulated substances without a license.
It's inevitable, if you have any significant number of clients. A pest problem went un-scouted until it was too late. An inattentive employee used the wrong stuff or used too much. The client either did not understand or did not follow your advice. Whatever the cause, something went wrong and the turf suffered.
Practice 'Root Cause Analysis.' Trace the problem all the way to the original, ultimate cause, and make the appropriate changes. Train the employee, correct the language of the advisory or bulletin, correct the mixing instructions. Then go back to the client, explain what went wrong and how you corrected it, and confidently explain what you're willing to do to restore their site to tip-top shape. This is professionalism at its best. You may still lose the client, but you are just as likely to earn their respect for your willingness to correct problems and improve service.
Some of the folks who hired you are quite satisfied, and may be willing to tweak their service in order to improve their site's appearance or performance even more. These are the people you need to contact right now. Explain what you think could be done to change their site into a real showcase, and ask if you could use them as a reference.
Just because there's snow on the ground or the clients' grass won't be green again until March doesn't mean that there is no work to do. Some of your clients are not completely satisfied, and you probably know which ones they are. Before they decide to hire someone new, call or visit them to discuss what went wrong or what could be done to improve their lawn's appearance. Some of these folks are going to blame you, and you may have to meet them half-way on the costs of getting their site back in shape.
Watering your lawn is something that, if not done properly, can do more damage than good. There are three basic concepts that you need to understand in order to irrigate properly.
First is timing. If you water your lawn on a regular, clock-work basis, be prepared to mow just as robotically and scout for pests that much more frequently as well. Watering when your grass needs it encourages drought resistance.
Second is the depth of moisture penetration. You need to moisten (not drench!) the soil to the bottom of the root zone. Between 4 and 6 inches, depending on grass type. In typical soils, one inch of water applied to the lawn will result in moistening the soil down 4-5 inches. Heavier soils will require more water, with some time with the water off to allow percolation. Sandier soils will require less water, but will need it more frequently.
These products are different than Weed and Feed products for other grass types. They usually contain Atrazine or Simazine, both of which are pre-emergent herbicides. They do not need to make contact with the weed plants, and they tend to last longer. Applying them anytime after the grass has fully greened up is Ok. Make sure you read and follow label directions carefully. If the fertilizer component of your product is NOT 15-0-15 or some other 1-0-1 formulation, DO NOT use it. Centipede can be killed by fertilizers designed for other grass types.