One of the most important personal protection skills we can develop is to set boundaries and clearly communicate them. Simply saying NO! in a firm voice is the most concise method of doing it.
Although the comment has been made that I give my clients permission to be rude, that has evolved in my mind to simply giving permission to say NO! As a society, we have placed so much emphasis on consensus and negotiation that our ability to firmly set boundaries and communicate them has become sadly neglected.
When someone refuses to respect a boundary that you have set very clearly, their intent becomes immediately clear. That clarity allows us to make definitive decisions about our own course of action.
Practice saying NO! every day in your normal daily activities. You don’t have to be mean or discourteous about it, simply say it firmly and with conviction. Often, when you say it, the other party will physically react and rock back on their heels. You have utterly reset their OODA Loop at that point.
tacticalprofessor 
I’ve had some unwanted attention from person(s) probing my vulnerability level. My response – “STOP! Step Back, Don’t Crowd Me! STEP BACK!” Meanwhile I’m moving away. The outrageously loud remarks from me, which could be heard a block away, solved the issue.
If an unwanted someone is approaching you, saying things like STOP, NO, or That’s Close enough, Bro, are vastly preferable to Can I Help You, which is sort of a carte blanche to keep approaching.
Find and watch Southnarc’s Managing Unknown Contacts clip isn’t the same as taking the class, where I learned things like that, but it’s a lot better than nothing.