The easiest way to think about guerrilla marketing is to compare it to guerrilla warfare. It's a small-scale endeavor focused on gaining ground against the enemy in small steps. The advantage is that you are leveraging your knowledge of the region (which the enemy does not share) to attack their weak points.
For marketing purposes, change "enemy" to "competition" and "ground" to "market share." Now we are engaged in a small-scale endeavor focused on gaining market share against the competition in small steps. The advantage is that we are leveraging our knowledge of the regions (which the competition does not share) to attack their weak points.
We take a different approach to advertising our product than using mass media or offering product discounts in our stores. We go where we know our customers are and communicate directly with them. A skateboarding company would advertise through a huge tag-style spray-painted design in a skate park (with proper permission ONLY) rather than buying a large scale television campaign. It's a less expensive advertisement, and while it reaches a smaller audience, there is far more retention among affected consumers.
Where are your customers? What would be an effective means of guerrilla marketing in your industry?