I have customers who have been with me over 40 years. For me, the key is prompt, professional service. Getting the job done. Being flexible sometimes and being willing to return to fix things even if they are not fixable. Being able to converse with a customer about why things are the way they are. Keep on learning.
You’re going to get a bucket load of posts saying what ^the Canadian said and they’ll all be right but staying in touch is most important. You can do a great job and wear flowers in your hair and they’ll still forget you if you don’t stay in touch.