It's good of you to ask this question before you head out. You'll develop a feel for etiquette as you encounter different fishing situations. Most of the time, it's as simple as giving others their space. When that's not easily possible (salmon fishing and some steelhead fishing come to mind), just respect those around you, and don't get too upset when they don't return the favor. Life's too short to get bent out of shape over a bunch of kegged-up, freaked-out, tight-lipped fish.
Here's a general rule that I apply to most of my fishing: If I'm fishing for steelhead and there's somebody else fishing the same water when I arrive, I always start fishing above them and follow them through the run. First crack at the water definitely gives you an advantage, but you can sometimes pick up steelhead while fishing behind others in the same piece of water. NEVER move in below someone fishing for steelhead (unless they come out and tell you it's OK to do so).
If I'm fishing for salmon, there will almost assuredly be others (often many others) fishing around me. In those situations, I find my own little combat spot and do the best I can. Here's where the patience of a saint keeps you from going insane.
If I'm fishing for trout and I see another fly fisher in the water I planned on fishing, I move to a different place on the river. My reasoning here is that once trout have been fished over, a given pool, riffle, run, pocket, etc. needs to be "rested" for a reasonable length of time (longer than I want to wait) before it will be productive again. Since fewer people are around on trout streams (at least the ones I fish), I usually opt to find my own water and let the angler who was there first enjoy fishing that stretch. I figure I can always fish that stretch another day. OR: If you are really set on fishing that stretch, take off for a while, do something else, and come back. Almost always, the water will be yours for the taking, and the fish will again be ready to eat.
When in doubt, ask questions. If nobody is around, you make the rules. SWEET!