I'm not even gonna write an opening for this post. You can take it or leave it. I don't give a f*ck! ;)
It is better to ask for forgiveness after than to ask for permission before.
I have a rule. I don't take pictures of someone without asking first. If he carries a gun. If not, I'll snap the picture without asking and deal with the consequences afterwards, even if a consequence would be to get my ass kicked
As a beginner photographer you are usually afraid of what will happen if you take someone's picture without asking first, and that someone "catches" you. But that is something you just need to stop caring about.
If you really believe in something, do it and don't let "nay-sayers" bring you down.
Many people don't like others being better than them. First of all, if you realize that you have some "friends" like that, ditch them. Don't spend time and energy on people that do not respect you and what you do. Second of all, don't be an ass: admit that others can be better than you and instead of being jealous, be inspired by them.
As long as you act like you are supposed to do what you are doing, the risk that someone will question you is very small
If someone acting like a waiter approaches you and asks for your credit card after you had lunch, you'd probably give it to him, right? And if a guy in a white coat would start doing CPR on a freshly fainted woman at a bus stop you'd most certainly feel at ease, believing he is a doctor and knows what he is doing. If you act confident and like you are supposed to do what you are doing, no one will doubt you. Behave like you are supposed to be where you are. Don't sneak around. Don't avoid eye contact. Try to act like someone that is actually allowed to be where you are.
Be partially blind
"Oh, was there a "Do not enter" sign on the door? So sorry, must have missed that! I´ll just go back (now that you caught me)…"
Just because something has been done in a certain way until now doesn't mean that you have to do it the same way.
The photography business is a crowded place. To be seen by the people you want to be seen by, you need to stand out. If everyone is doing something to get in touch with the right people, you should do something else. If everyone else is zigging, you should be zagging!
Everything is allowed until someone tells you it's not (as long as you are not killing anyone or robbing a bank)
If there isn't a sign saying you can't take pictures, you can take pictures. Jump that fence, open that door, walk behind that doorman when he isn't looking. Unless you are stealing, destroying, punching or breaking something or someone, what could happen? You'll get some loud words, certainly be called "bird names" and a "don't do that again!" shoved in your face but that is probably all. But you got that really cool picture.
Go with your instinct.
It is probably the right action to take. If not, it probably won't kill you anyway. If it does, I´m not taking responsibility for it. Sorry.