Wow, that's a big question. It's hard to distill down everything you learn about starting a design business into a couple of points. It's also hard because every person and every business is different. The things I've done may be totally wrong for you and your business. But I'll start with one point...
Keep your costs down. The more you save the more time you can take off or the less pressured you are to bring in work. Having a huge and beautiful monitor might be great, but you might end up having to work longer hours to pay for it. It's easier to control costs than it is to control how much your clients are willing to pay.
It'll be interesting to see what other people might say.
Don't spend money on printed advertising etc.. The cost far outweighs any benefits. Don't be afraid to say no to people. There are some real f**kers in the world who'll expect the moon on a stick at vistaprint print prices. Don't feel the need to give every client a discount. They won't appreciate it and as soon as the discount ends, so will the business relationship. Besides, every hour you spend working on a cut price job is an hour you're not working on a full price one. Don't put all your eggs in one basket. It's all very well a client saying they're going to give you £2k worth or work a month but actually getting it is a very different story.
Do spend time on social media. It's a free and easy way to get your business out there. Do read loads of business blogs. There's loads of really good advice in them! Do ride on other peoples coat tails. If you've worked with a prestigious brand, make sure people know.