You don't.
I do it because I want to. Whether someone wants to take advantage of my goodwill, so be it. But with every lesson u learnt.
Obviously u will feel disappointed when the negative happens to you. But whenever there is a positive reaction, then u will react more positively.
You will also learn to improvise on how you would react.

Let me quote an example:
I usually preach that I won't donate to any foreign beggars regardless how pity they look. But damn I have a soft spot for children especially mom with baby.

You know it's cartel and giving monies just encourage it further. So instead of donating, I bring them to the closest shop or restaurant where I can get them food. (eg ramly burger, 7-11), buy them a meal that will sold their hunger for that day.

Could I be scam for this? Yeah maybe but that's life.

You be you and be happy with whatever decision u make and not because other influences you.