Having assisted my wife with opening her business, and running it for about 6 years (modestly successful), and being a contractor myself in the past for many, many years (very successful), my advice items are:
1) get iron clad documentation and contracts for any and everything that you do
2) if you are partnering with anyone, make sure everyone's roles and duties are clear, and that they are capable of fulfilling them. business is business. friends and family are potentially awesome employees and partners, and also potentially the worst thing ever. be able to make rational decisions on people's ability to be an asset, not emotional ones.
3) if you working out a start up budget, price all items at worst case scenario levels. Then double that figure. I'm not kidding. having leftover money is always better than coming up short.
4) when working out time frames, see item #3. when offering something that has a timeline to a customer, this is especially useful. if you finish ahead of schedule, you look like a total hero. being late makes you look like a zero, always.
5) choose partners, employees, vendors, sub-contractors, and 3rd parties (such as book keeping) wisely. no one is in a better position to rip you off.
6) plan on not making much, to any, money that will go beyond living expenses and overhead for 2-5 years.
7) plan on working 12-18 hours a day, every day. this may or may not include networking. always grab an opportunity to promote yourself/business in a social situation. if you don't engage in many social situations, start.
8) be aware that YOU now shoulder the responsibility for every little thing, from buying pens at staples to getting woken up in the middle of the night being told the water heater has broken in your store and flooded it.
9) advertise in manner suited for your business. Facebook is a helpful additional tool, not your primary advertising. Don't put an add in the yellow pages unless your target demographic is over 65, otherwise it's a waste of money. Don't bother building a website if there is no reason for someone to visit it (like if your clientele comes almost entirely from word of mouth, or if you're in a service business with an existing customer base). if you do build a website, it had better look good and WORK.
10) don't put advertising on your vehicle if you drive like a dick. if you do put it on your vehicle, always remind yourself it's there. drive accordingly.
11) do your homework. if someone suggests something for your business, take it with a grain of salt, and research the idea if you're interested. Most times it will turn out to be not worth it.
12) if you are planning on having a storefront, workshop, warehouse: location, location, location
13) don't confuse being firm with being a jerk. always be nice, but don't take any $hit. you and your business come first (when you're doing business).
14) conduct yourself with the highest standards of ethics. do what you say you will, make sure that any wrongs are always completely righted, and that you are just plain honest. nothing will earn you more respect and clientele than fair, honest, complete and quality service. you are the business and it is you. act accordingly.
Good luck! I hope that your new business is a great success!