I have my own IT consulting company that I run as a side business (I have "real" full-time job as well).
I make enough money from consulting to pay the payment, insurance, and never-ending stream of modifications on both a 3-series AND a 5-series BMW, and to pay for my insane computer habit. I've been in business for a little longer than 3 years.
Being a reseller with a store (either B&M or on the web) is no way to live. I do sell anything and everything computer-related, but I buy it from newegg or elsewhere and then mark it up (a lot

). I NEVER have anyone come to me "just" for a computer. There's always some problem or special case that needs to be resolved. If they want just a computer, I tell them to call Dell.
I have 5 core small business clients that rely on me to keep their computers/networks/website/etc running. In addition I have about a dozen other "regular" clients. So far all have been obtained by word of mouth and recommendations from other clients. I've never had a client that I've only done one job and then never hear from them again. I have done zero marketing. None. I have a web site but it has been neglected for nearly 2 years and no one has noticed.
A lot of it has to do with your personality. If you are not people-oriented, you will fail as a consultant no matter how good your technical skills are. It helps immeasurably if you can explain technology so someone who's head is filled with rocks can understand it. And of course if you can figure out weird problems quickly that helps too! You have to know a lot about a lot of things, or at least know where to find answers while not looking stupid in front of the client.
Many small businesses have been burned badly by greedy and incompetent "consultants". I've found that being up-front about costs and potential problems goes a long way. As does occassionally working for free on those really thorny problems that take longer than expected. That builds a lot of client goodwill, and is the kind of stuff they tell their peers about, and makes them happy to pay their bills.
Bills - that's another thing. There's a LOT that goes with having your own business besides doing the work. Getting people to pay, or having the confidence to convince them to pay when they really don't want to, that can be difficult. At first I hated that part, many people are not comfortable talking about money - especially asking other people for money. Now I don't mind at all. And taxes, and all that other stuff. That's harder than the technology side of the business! I'll say one thing - it's never too soon to start learning a little about accounting, taxes, insurance, and all that other stuff that goes with running a legitimate business.
Well, I rambled on a little long here, if you have any specific questions post them or PM me if you like.