As someone who's worked for several SMB IT Businesses (that failed) i can offer you up two very VERY important bits of advice.
1. Be 100% customer oriented, regardless of how small they may be. Losing a small client can have a big impact on your bottom line, and you'll find it's hard to get clients in the first place, so do your best to keep any many of your clients as possible.
2. Never be afraid to rely on vendors for help/contract work. You'll end up needing to do a job that you've never done before, rather than do it wrong yourself, sub-contract out someone who you know can do the job. If you build good vendor relationships you can easily make this affordable.
As a field engineer i've seen both of these ruin businesses, you'd think they would be simple enough.
~From an Anonymous Blogger~