I've become suspicious of self-help tactics, not because they don't work but because few people have the discipline to execute them. For example, the concept of "sharpen the saw" (from The 7 Habits of Highly Effective People) is excellent advice... that probably 1% of readers of that book actually turn into a habit.
Behaviors that become habits are generally both easy to execute and have an immediate pleasurable payback. That's why people become so easily addicted to drugs, TV or video games. By contrast, "sharpen the saw" neither easy to do (it's hard work!) nor particularly pleasurable; hence it's a substantial challenge to make it habitual.
Most of the habits that can be acquired easily have either a negative effect (like drugs) or a neutral effect (like TV) on your life and career. Habits that create positive, long-term change (but are easy to acquire) are rarer, so rare, in fact, that most people are pressed to name even a single example.
Not to worry. I've got one for you.
Consider your mental process when you see another person whom you don't immediately recognize. Your first thought is automatic because it's genetically programed into your brain. It is the question "friend, foe, or neither?"
If your brain tells you that the person is a (potential) friend, you might approach them. If your brain tells you that the person is a foe (potential or otherwise), you go into "fight or flight" stance. But most of the time, your brain says "neither," because most of the strangers you see simply aren't immediately relevant to your world.
The thoughts you have in the third (and most common) case determine, in many ways, how you feel about people in general. Most of the time, you simply dismiss them from your mind. However, it's also very common for people to immediately formulate a criticism of the other person, like "he's fat" or "what a loser" or "what an ugly hat" or whatever.
The problem with such uncharitable thoughts is that they create a sourness in your attitude when dealing with other people. It's very much like suffering from a negative internal dialog; if you keep thinking "I'm stupid" you do stupid things.
Similarly, if you continually criticize other people (even if only in your mind), you start treating other people poorly. To make matters worse, other people intuitively sense your negative attitude and respond by treating you poorly. It creates a self-reinforcing cycle that makes you, and everyone around you, more miserable.
There is an alternative, though. Rather than dismissing or criticizing when you see a stranger, bring a kind thought or a positive idea into your mind.
For example, while driving down the street you see a homeless person. You immediately realize "not a threat" and then either ignore him or, worse, judge him or her.
If instead you think something like "gee, I hope he's all right" or even "I'll bet there's a story there," you're training your mind to think the best of people... even people you'll never meet. This attitude quickly permeates your interactions with people whom you DO meet. People will sense that you're a kind who wants the best for other people. You'll treat them better and they'll treat you better.
As I mentioned earlier, this habit is extremely easy to acquire because 1) it's very easy and 2) it has an immediate payback, which is that you'll feel better about yourself.
How to acquire the habit in 10 minutes
Next time you're in a public space or around a lot of people, set the timer on your watch or phone to 10 minutes. During that time think a kind thought about everyone you see. You will be surprised at how more positive about yourself and others you'll feel at the end of that 10 minutes. What's more, it's likely you'll enjoy it so much that you'll find yourself doing it automatically, going forward.
I can say without reservation that acquiring this habit has changed my life for the better. I used to be hyper-critical of other people and the attitude it created in me both made me miserable and, frankly, a bit insufferable. However, since I changed my automatic thoughts about other people to consistently kind, I've been much happier dealing with other people and people treat me better as well.
Seriously, try this! The most you have to lose is 10 minutes of negative thinking and what you could potentially gain has immeasurable value.