Before I started my welding shop 18 years ago, I talked with a lot of self employed people. Most of the stuff I learned way back then still applies today, maybe even more so then it did.
1) No employees! If you can't do it by yourself, you won't make it in the lean times. Hiring employees makes you a time manager, you manage the time they are suppose to be working, and you manage the time for you to secure work to keep them busy. If you hire employees, they need to produce income 2X what you are paying them, to break even.
2) A written contract on every job detailing everything you will do, the amount and time frame payments are to be made, and what happens if the payments are not made on time.
3) Get paid for any parts/ material your going to need up front, and use that money to buy the stuff you are going to need. If everything turns to crap, at least you don't have a pile of parts to pay for as well as loosing all the labor.
4) If whatever you are doing is going to take more then 2 weeks to complete, set up a weekly or biweekly payment schedule for the labor. If the customer gets behind on the payment schedule, all work stops, and a predetermined plan takes over.
5) Keep track of your hours on each job, use a time clock to stamp each start and stop time, document what was done every hour during that time. Picture proof is helpful if you go to court.
6) Finance as little as you can, monthly payments suck for a new business. It takes 1-3 years before the business can carry itself, and another 2-3 years before it will generate a minimum living wage for you. Don't spend more money then you make!
7) With the current tax laws, you might have some pretty nice stuff, but very little in cash reserves. The government (local, state & fed) are really good at taking cash reserves away from you.
8) Hire a good accountant.
9) Talk with a local to you lawyer to find out everything you need to legally have up front for your business to open, and what you will need to keep functioning in the future for the business you go into.
10) Set your business hours, and stick with them. Anything before or after hours is extra cost, no exceptions. If someone has an emergency and needs you after (or before) hours, they should expect to pay a premium.
11) There should be no discounts for family or friends! Your expenses don't change while you work on their stuff. Family and friends should understand that.
12) Set your hourly rate competitive with like businesses in your area, not cheaper nor more expensive. Be sure to charge for each hour you work on something, or charge according to a time rate for the job you are doing. If you can't make money on the time rate, you either need to improve your performance, or look into ways to improve your time.
13) If you don't enjoy what you are doing, why would you want all the hassles?
14) if you sell your business, DO NOT become the bank
