According to Wikipedia, Guerrilla Marketing is described as:
The concept of guerrilla marketing was invented as an unconventional system of promotions that relies on time, energy and imagination rather than a big marketing budget. Typically, guerrilla marketing campaigns are unexpected and unconventional; potentially interactive; and consumers are targeted in unexpected places. The objective of guerrilla marketing is to create a unique, engaging and thought-provoking concept to generate buzz, and consequently turn viral. The term was coined and defined by Jay Conrad Levinson in his book Guerrilla Marketing. The term has since entered the popular vocabulary and marketing textbooks.
Guerrilla marketing involves unusual approaches such as intercept encounters in public places, street giveaways of products, PR stunts, any unconventional marketing intended to get maximum results from minimal resources. More innovative approaches to Guerrilla marketing now utilize cutting edge mobile digital technologies to really engage the consumer and create a memorable brand experience.
In other words, you need to do something unique in such a way that people are going to be talking about it for a long time. In my opinion, the T-Mobile campaign is probably one of the best in recent memory; paid actors simply filled the train station and in unison began dancing a choreographed routine. It was remarkable really, to the extent that individuals not paid to participated started dancing, have a look.