2. Learn to play the sales and marketing game. As engineers, we often underestimate the value of presentation. Showing up for a meeting well dressed and groomed has a really positive effect on a prospective client. I used to send my proposals as plain text files. People started taking me a lot more seriously once I switched to PDFs with proper fonts and colours that reflected my own company branding (website, stationery etc.).
3. Get money from your clients when the time is due. Don't become a line of credit for them. I learnt this the hard way and have a significant amount of money that's due to me which I'm not hopeful of getting.
4. Learn to evaluate a potential lead quickly and then decide on how much time you want to spend on making the proposal. Good leads might be worth a quick prototype of the project even. Bad leads might not be worth the email you send telling them that you're not available. Your time is valuable and it's what you're selling. Spend very little of it for unpaid tasks.
5. Put an expiry date on your proposals. Tell them that you'll do X for Y $ if the project starts by Z. Don't skip the Z.
6. Charge an advance before you start the project and charge in pieces based on value delivered. This also helps your cash flow.
7. Structure your company properly. A good book on that topic (and several others - a must read IMO) is https://davidmaister.com/books/mtpsf/
8. Keep growing. In my experience, consultancy (the services business) is a numbers game.
9. Spend some time, money and energy setting up policies for HR etc. up front so that when your employees come about raises and things, you have answers ready.