It of course depends on your business and business model, however, in my opinion, the best method is, what in Russian we call sarafanaya radio (velvet radio), in other words, person to person, word of mouth.

Customer opinion can either make or break you and if you are in a tight related industry: for example an oil-gas services company, you can count on the fact that many of your customers know each other and meet at the same conferences and or events or may be personal friends and yes, they talk and gossip.

Get people to experience your product, your service and they will spread the word as effectively or more so than commercials. Commercials are good to get your name out and get the initial customer in, but its personal testimony that does the main work and solidifies or ruins your reputation.