The very first promotion steps
The ways to promote your products and services are reviewed in Marketing and Advertising in Second Life section in details.
This article covers the "must do" steps you have to perform when you launch your own SLbusiness.
In-world
SL Search
List your parcel in Second Life search by checking Show in search option of the parcel details. It costs L$30 per week, but you have to have it turned on.
SL residents use SL search to locate new places as well as to find the parcel they want to visit again. They won't be able to find you if you exclude your business from SL Search.
Landmarks
If you sell out any objects, put a Landmark for sale along with the purchase. This will help your buyers to locate your store in the future, and also helps them to recommend your services to friends.
Your profile picks
Add a pick to your profile pointing to your business location, ask your friends and coworkers to do that as well.
On web
Official SL blog
Create a blog entry in Linden's official blog to tell everyone about your services. The effectiveness of such entry is very low, but it's free, it will be indexed by Google and may give you some customers in the future.
Official SL forums
You may also want to post a message about your services at Second Life forums.
Don't have a website?
Having a webpage is absolutely necessary if you run a business in SL.
But you don't have to create huge corporate portal or register a personal domain name.
The small blog works out of fine if you want to present your goods and services. A piece of text and shots of the most interesting of your goods will be enough to attract the potential customer to visit you in SL.
Create blog at SL Universe or any other free blog hosting and you get your own link to advertise.