Some of these tips apply specifically to fly fishing but the principles of consideration for others and self-awareness should be practiced in all forms of fishing and indeed any other country sport.
The onus of etiquette is on the seasoned anglers who have the know-how for tricky or crowded spots, to be patient and respectful to more novice anglers, try
to impart some of their knowledge to those beginners and try to make the whole experience more enjoyable for everyone.
First and foremost, etiquette starts with the fishing guides; no matter how many spots in however many countries you’ve fished or how long you’ve been fishing, when you travel to a different country or even a different part of your own, local anglers may have traditions or customs that you would have no of knowing about so it is the responsibility of the guide to make you aware of these before you start casting or even picking an area. This is assuming that you have a guide with you, if not try to call on your own knowledge and ask the other anglers near you if you’re unsure of anything, but remember that they may not want to be disturbed and you must respect this too.
The most important thing when enjoying the countryside is to leave no trace of your activity there. Which means to leave a place the same or better than when you arrived. Always check that you take any rubbish or excess line with you. And if you're a catch-and-release angler, do your best to ensure your catch will live once returned.
When it comes to fishing spots it's usually first come first serve. It makes no difference who fishes a part of the river more or can potentially fish it better, whoever got there first has a claim to the spot until they clear out. Recognise their fishing space and it should make for a more pleasant encounter for everyone. Now, this does have an exclusion - crowds. Sometimes there are just more rods than there are spots to fish, so fishers will have to cast with tighter spacing. This is usually seen with fish returning to their spawning waters. When you find yourself in such a bunch, the person furthest downstream should cast first, followed by the next person up, and so on. If you’re unsure if you are encroaching on someone’s area the best thing to do is ask if they mind you casting or crossing the river near them etc. Correspondingly, don't install yourself in one run all day. It might be the most fruitful spot on the river, but manners dictate that you share with others.
It's also best to keep your presence to a minimum. A number of fish species spook exceptionally easily and any sudden splashing, shadows, or noise can cause them to panic and either take off or not bite. This doesn't mean you have to be completely silent or can't have a conversation, but it is worth taking note of. It also helps to be discreet so other people near you can enjoy the outdoors; your voice should only be heard by others when they actually want to hear it. It's great to be enthusiastic about fishing, but that doesn't mean hollering about and disturbing everyone else’s experience. It’s also important Keep your dog on a lead (or leave your dog at home) if it's not the kind of dog that will heel right next to you as you fish.
If you want to fish on private property you need the land owner’s permission or you will be trespassing. Private property is defined differently by country or region, so make sure you know the regulations for where you’re planning to fish. If you aren't sure if it's private property or not play it safe and don't go on it.
Occasionally the selfish or uninformed cry foul with no good reason at all, so as long as you allow appropriate distance between other anglers and yourself, and don’t unnecessarily disturb the area, don’t be put off by a grumpy local who think they own the river.
Lastly, if you feel like someone has done something wrong that adversely impacted your fishing, don't shout at them, flip them off, or get into a squabbling match. Odds are, they don't know what they did. If you can calmly let them know, and then move on by saying "no problem" or something to that effect, they'll be smarter, you'll feel better, and a bad encounter in the future will hopefully be avoided.
This guide is by no means comprehensive but it’s a good place to start for beginners and it all boils down to common sense, just like most aspects of fishing.