What is Guerrilla Photography ? Something I just made up ! Needless to say, there are no formal rules.
Maybe later, once it's taught in art class and becomes old school and boring. But for now, here are some general rules of thumb:
1. Travel light
No heavy artillery, like SLR cameras with foot-long telescopic lenses and supporting tripods.
Powershots and Kalishnikovs. Point and shoot. Ask questions later.
2. Get off the beaten trail
Any sign saying "Trail Closed", "Area Boundary", "No Trespassing", "Warning..." is an invitation to adventure !
What are they trying to hide from you ? Respect your neighbors, respect the environment. Not mindless authority.
3. Hit and run tactics
Don't spend hours on a viewing platform setting up a preconceived shot and waiting for that perfect moment to pull the trigger. Go wherever the spirit moves you and take what nature gives you. Be ready to strike at a moments notice.
Master the one-handed "blind" (no viewfinder) shot while hanging from a treebranch or dangling from a rope.
This also comes in handy when you want to put yourself in a shot and there isn't a good boulder nearby to balance the camera on (as above), let alone, a helpful Chinese tourist eager to trade group portrait-taking duties !
4. Quantity, not quality
Take multiple shots with slightly different focal points. Let fate decide which exposures turn out best.
Remember, there are no "bad" shots. They are all raw material on which natural selection can operate.
And if not that, there's always Photoshop !
5. Consider the objective
Are you aiming at getting published in National Geographic ? Selling framed and signed 3x4' laser prints at an art fair ?
Then please ignore all of this (as I'm sure you already have). Are you going to view the results on a PC monitor, TV, iPad, iPhone, or similar ? Can you really read between the pixels of your given display device and observe the subatomic detail that makes up your 100MEG original ? More to the point (since the technology to do that probably exists): do you really want to ?
6. Remember to Be Here Now
From time to time, put the camera back in your pocket (or let it float over your shoulder on its lanyard).
Try to forget about it. Experience the world directly, not through a tiny 2-dimensional LCD screen.
Ahhh... that's better...
Damn ! Look at that ! OK, maybe just one more photo...