So, having started from a shed in my mum's backyard, and building $1,000 websites, and growing that without investment to 200 people. My biggest two websites builds at that later stage were $1.5M so I'm probably qualified to answer this question :-)
Firstly I wrote a whole book on the subject called Agencynomics, so I'll keep the answer less than 250 pages for you!
The first step is to understand that you need to get the business to a position where one person is designing/building and one person is selling/account managing and project managing. To do that you need to get your billings to a level where you can afford two salaries.
When I started, I was selling all day and designing/building all night or vice versa, but I sold $45K dollars in that first year and managed to recruit someone, once I got to about $6K a month, as a result, the next year I grew to $150K, with that second person. I was able to charge for Account Management/Project Management/Strategy from me and Design and Build for my employee. The key thing was to keep adding in value-added lines, which slowly bought the price up, like UX, SEO optimisation etc. Today websites are commoditised so maybe a shift to focus towards progressive webapps (PWA) or other more scarce services will help you drive up day rates.
What I found was and still do today, in the early stage you give away the thinking planning, strategy, and small clients don't want to pay that much either, but as you grow you master selling better.
Try not to sell to price buyers, but to clients that are relationship or value buyers, people that appreciate the service. Try not to overdeliver, underpromise and overservice, but not overdeliver! Scope creep from clients makes it harder. So be careful. The truth is as you scale you naturally get better at pricing and the projects get bigger and you get more confident in your pricing. But if you play in the "many providers" market of websites, the prices are going to be commoditised. So think about the design and build problems you can solve that are scarcer and therefore people pay a higher premium for and can't get your service everywhere. Lastly, set yourself targets. Each month, everytime you hit the target move it up and keep pushing. This is the best way to scale faster in the earlier years. Everyone forgets to set targets in those early days.