This is not my advice - I'm 26 - but was given to me at school by a now-retired teacher I greatly respect. It sounds quite wise, and I trust the source.

I paraphrase: "Learn to classify people into three categories: givers, sharers, and takers. Surround yourself with givers. Sharers are also acceptable. Cut out the takers as quickly as you can."

A verbatim follow-on quote: "Takers make a beeline for givers. The needy are always anxious to drain the emotions and finances of those who are givers, before somehow or other they move on to sponge elsewhere, leaving givers to wonder at their own foolishness."

(Edit: apparently people don't want advice from older people to younger people if it's not the older person giving it. I can only apologise. It's the primary piece of concrete actionable life advice I remember that was given to me by anyone more than three times my age.)