Under promise, over deliver. (slightly)
Become an authority on your subject matter. Write blog posts, post often on twitter, write to journals/newspapers - set yourself up as someone who knows what they are doing.
Charge for spec work.
Don't be afraid to turn down work that may challenge you more than you can handle. You are taking an unnecessary risk. In those circumstance you can offer to help the client find the right consultant/solution provider.
ALWAYS get a deposit before work begins. And then insist on regular payments for longer projects - if payments stop, you stop work.
Don't do fixed fee work, unless you REALLY, REALLY trust the client.
Good relationships are vital. If a client likes you they will market you through word of mouth and that's the best kind of advertising.
Have a plan in place to deal with non-payments or difficult clients. Be consistent and don't let bad clients take advantage of you. Offer no credit terms longer than 28 days. Offer a 5% discount for quick payment.
If you are providing software before full payment is received, don't be afraid to use a licensing mechanism to shut it down if payment fails to materialise. A friend lost €20k in a similar situation.
Also, bear in mind you will work long hours. That's why I couldn't keep doing it. I was making decent money, but the hours, and travel, were killing me.