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.