Let's look at a general example.

We are a company that provides services for organizing events and building exhibition stands. We have a client who wants to order a stand from us.

1.    The first thing you should pay attention to is establishing contact (politeness, a smile in your voice, and the human factor, which makes it clear from the first minute whether communication is going well or not).
2.     The speed of response to an incoming request, to messages, and to further customer requests. After analyzing competitors, you can find out the average time for a call back to an application, usually from 5 minutes to several days, the main thing is that they call back at all (sometimes they forget). It is also very important to respond to emails as quickly as possible. Usually, we are used to responding in messengers faster than through email. So we exceeded expectations here as well. I'm sure the client will note this and be pleasantly surprised.
3.     Always say yes. Even if the client asks you to do something impossible or inconvenient, you can agree at first and then politely get off with a joke. Such a response will be perceived by the client as less stressful than a direct refusal of their request.
4.     Keep your word. Be responsible for your words, for your promises. When you say it, you do it! This is one of the most important rules.
5.     Meet deadlines and stay ahead of them. Life hack: if you are not sure that you will meet a certain deadline, take time to spare so that you can do the work faster and impress your client again.
6.     We are all human beings and we are prone to make mistakes. If you make a mistake, you need to admit it. In no case should you shift the responsibility to the client. It's important to find a way to make amends with a nice bonus and a correction.
7.     Quality of work. Every company should have this item by default, but in reality, this is not always the case. Even if all the points are done perfectly, and the quality of work is not the best, it will be very difficult to regain the client's satisfaction. So do your job the best you can. This point is one of the most important, because it is the quality of the service provided that will be used to judge your company as a whole. The result of your work is the face of your company.
8.     If everything is done perfectly and the service is provided with high quality, you can pleasantly surprise the client upon completion. Thank them for the joint, successfully completed project. Present him with a thank-you note in which you highlight the key points in the work. You can also make a symbolic keepsake to strengthen the working relationship and exceed expectations even further.

Many companies don't follow this strategy, or they know it but don't do it. You have the simplest instruction, which is easy to implement and does not require any special investments.