Business capital. Without it, you are destined to fall behind the curve, run out of money, and give up or slowly die due to lack of capital. Put away 6 months of your operating expenses at minimum, which includes your salary, advertising, insurance, phone, expenses, etc. You can forgo things like retirement contributions, profits, etc, for a short while, but eventually you want to make sure those are part of your overhead as well.
Advertising and marketing. Crucial. Not only will it cost money, but suck up a tremendous amount of time. Pay a good designer to help you with your marketing materials and web presence. Its worth it.
Business courses. Run it as a business from day one. Count everything you do with any link to the business as your overhead. From the paper clips you buy, to the time you spend doing research on your competitors. Its all time and expense the company needs to cover, and not come out of your pocket. If you don't count it as overhead, it will come out of your salary, and you'll end up wondering why are aren't making any money.
Price according to the market, your cost structure, and your target clients budget. If you costs are higher than what your clients are willing to pay, change your clients, not your prices!!! There are hordes of people out there willing to pay $2,000 for a portrait sitting and some large prints. If you target your marketing to them, and have reasonable photography skills, you'll eventually carve your stake in that end of the business. If you just lower your prices and cut your costs, you could end up forever struggling with $50 portrait sittings and $15 8x10s.
Match your skills to the market segment you want to work in, go after them and ignore the rest. If you determine you should be the $250 sitting, $45 8x10 print range, don't accept the $50/$15 jobs that fall in your lap. Instead, use that time to continue marketing to bigger fish, rather than spend your time reeling in a smaller fish you will eventually throw back in. Its time wasted on a short term client.
Make sure you cover yourself as a business. Register as a business legally. Make sure you have liability insurance so when you drop a 800ws light on a toddler, you won't lose your house and life savings. Pay your taxes, keep on top of the financial and legal aspects of the business. Register your copyright for all the images you shot over the last 90 days, and do this every 90 days.
Understand the market and how it conducts business. Each segment is different. Creative fees, contracts, and usage/licensing are common in commercial work. Booking/Sitting fees and print prices are traditional in portraiture and weddings. Day rate vs space rate, one time serial rights, reprint rights, etc, are common in the editorial market. Don't make assumptions, rather take the time to understand and make informed business decisions.
Join organizations, forms, and networking groups where photographers in the same market segment are abundant. You will learn a lot. Photographers and creatives naturally tend to share information, more so than in other business segments. Don't know why, but its true. So take advantage of it.
I'll think of more later, and try to revist. Time to share a Manhattan with the wife and watch a movie