When first starting out its a good idea to start with people you know and then work your way out from there. Market to them, let them know what you can do and that you are commencing on building your business. Get testimonials from any clients you already have or past employers or co-workers. Make sure you have business cards and a flyer.
I visited people I knew to make sure that they told others about me and it grew from there. Just like job hunting, it pays to door knock. After a while you move onto word of mouth and so on...
I had another thought... call all the places that advertise for part time casual people as these jobs generally can be done by a VA. Explain what the differences are to them and how much easier it is to have a VA rather than an employee :)
I just did a four part series at Home Office Warrior with tips on marketing. I compared finding clients with fishing and asked for ideas from others on ways to 'reel in the big one'. You can find the series here. Home Office Warrior Virtual Assistant. I hope you find them useful.
The first thing I did was join my local Chamber of Commerce. You would be amazed how many people are interested in the VA field, simply because they may have heard about it, but never utilized it. I find local networking the best ad. Good luck!
1. Identify your target market. Who do you want to work with? "Anyone with money" is the wrong answer.
2. Identify yourself ;) "Optimal Admin" is a good business name, but we'd love to call you by your name ;)
3. Make a list of things you've done, and rate how successful they were, and how much time and effort they took. Sometimes we forget to go back and see what worked and what didn't.
4. Work free. Find folks who are your ideal client (see #1 above) and offer your services free for 2 hours to show them what you can do. Tell them the only cost is honest feedback on your work, but that you'll accept a testimonial if they're willing to share one. Don't make the testimonial mandatory or it pollutes the authenticity of their comments. Testimonials are solid gold when you're looking for new clients.
5. Blog. Share your expertise in your specific niche on a website. Giving away information which will be helpful, whether folks become your client or not, creates enormous good will.
6. Have a specific niche. You cannot be all things to everyone. Specialists are more skilled than generalists, and get paid better, too.
With more details about what you're doing, and more information about what you've done regarding these 6 steps, we could probably offer more help.