Your best bet will be through referrals - generally people you already have established a relationship with in past jobs, etc. All of my initial clients were former employers.
Advertising will work as well, but it will be expensive... and you don't have a name or reputation yet, so you'll still have to work to get people to trust you.
ranisavljevic, Who is your customer? That's an important question to answer. I'm in a business that is "sports" oriented. It's always surprised me that business' similar to mine advertised in the local newspapers in the sports section. The reality was/is the large majority of buying decisions for our service were initiated or made by the female of the household....different section of the paper. Shhhhh. Bottom line, know who your customer is. btw, no offense meant to any female readers, those were the numbers.
Present day: In the past couple of years. We've diverted our advertising budget from yellow pages, newspaper, etc. to investing in our web presence and email marketing. Most recently we've invested our time and learning opportunities into knowledge of SEO and inbound marketing (coined by Dharmesh I believe) concepts. I agree that referral based marketing is one of the least expensive methods to grow your business, but you need customers first who can refer you. I recommend visiting http://www.hubspot.com ASAP and take advantage of all the free information they have to offer. They have a lot of valuable information. Once you've gone as far as you can it may be worth your effort to invest in their services.
Best of luck to you in your new venture.
Referrals are pretty much key. Just one seasonsed sales guy can have a rolodex of over three hundred people who are connected to another three hundred each. You never know where you might people: in the grocery store, in an elevator, on an airplane... Everyone has interesting stories and you never know what might come up from random events! Viral marketing, network effect all pretty much rely on this phenomenon so use it!
As for advertising, Dharmesh talks about Outbound vs Inbound Marketing which might interest you a lot.
Another way is by offering your services at substantially low prices to a start up and building that relationship ....it is a win win for start up and yourself .. Startup wants something done for very low price , and you are looking to join the start up in the long run .... THis could avoid all the complexity of contracts related to equity .. of course only works if you are really committed to working on low $$ and startup has that $$ to pay ..