The best advice I can offer is this: -
1.) Present yourself as knowledgeable and professional. If you DON'T know something, tell the customer you will find out and get back to them. Don't bull**** people.
2.) When you promise to do something, do it. Get quotes to people on time, turn up to jobs when you say you will. If you can't make the odd appointment, call and let people know.
3.) Have a 'target customer'. You may think this sounds stupid but people calling plumbing companies are after varying levels of service. Many want the cheapest service possible as they are skint (sadly). Some want a quality service and are willing to pay a premium for this. The latter people are the ones who will help you grow your business, but only if you deliver what they want. They will be the loyal ones who will keep using you.
There is no point in being the cheapest as the customers who want the cheapest plumber are not loyal. They will phone around next time they need a job done and find the cheapest again.
This is really important and imo is the real art in running a business. You can waste a lot of time quoting for people who either: -
A.) Don't want the product your business offers.
B.) Can't afford what they want.
C.) Have no intention of paying for what they want.
It's taken me longer to suss people out than it has to learn how to be a plumber. Generally now I have a good idea if I am going to get a quote within 10 minutes of entering the property. Not always mind, sometimes I still get surprised!!
4.) Don't pop over to quote for jobs that are less than a day in length. You should be able to give a reasonably accurate price on the phone by asking certain questions. This gives you more time to promote your business, do admin etc. It's worked very well for me. If the customer doesn't want to pay the money, it's been a waste of 5 minutes on the phone but it's not cost me any fuel or an hour of time to go and quote.
5.) If there's a disagreement or an issue, try and see it from the customer's perspective. This wasn't something I was great at doing initially. I lost a couple of customers in my time because I refused to see things from their point of view. Recently I've kept a customer who will probably be a regular one because of adjusting MY attitude.
6.) Keep a tight control on costs, even as your business grows and expands.
7.) Get the right systems in place when you start your business up that allow it to grow. Think particularly about the company name. Dripbusters Ltd was great for plumbing. Now we do electrical work (and gas soon) it's not so great. You can push through with a name that's not great but you have to market yourself a lot more.
8.) If you do all of the above consistently your business will grow and you should be more and more profitable each year.
My biggest blocker to growing my business initially was me. I (actually my wife) was very honest and said that I came across as very stubborn and abrupt when a customer had an issue. I'm always going to be like that deep down but stepping back to see things from other people's points of view has really helped me grow my business.
9.) Never get complacent and stop marketing your business. Many plumbers say they don't do marketing but I would challenge them and say they are missing a trick. Ok, so you have to take more staff on once you exceed what you can do in a week but what's wrong with growing the local economy? Our business is 6 years old and our apprentice always flyers each estate we do a large job in. We do this despite having an excellent online reputation and getting repeat business from many regular customers. It always generates more work and customers. As the business grows it needs more orders to sustain itself so you can never get complacent.
10.) Starting a business is very scary. I earned £3,000 in my first year, working 60 hours or so each week. This is my 6th full year now and after our wages (mine and 2x staff members) we've made £20k profit for this financial year so far and we aren't even a quarter of the way through it yet. I've massively exceeded my original expectations of what I thought was possible financially. The exciting bit is there's still another 20 or so years in me yet I reckon, although not that much on the tools!
I'm certainly not exceptional so if I can do it anyone can. Best of luck to you!