Avoid significant relationships (of any kind) with drama llamas, emotional vampires narcissists and bullies.
Keep drama addicts, narcs, toxic people and psychic vampires as far away from you as possible.
Agree with PPs that this is so important but before you can do this, you have to know exactly how to recognise drama llamas, toxic people, psychic vampires etc.
Many people who were raised to be people-pleasers, peace-makers, who were told "don't be a show-off/judgemental", were taught to "be kind" and always put others first etc – in general weren't taught how to stand up for themselves, know their own worth and what they want and need, and actually don't know automatically how to recognise toxic people. The learning often comes by way of an extremely harsh experience of becoming enmeshed with a narcissist or other user.
So if you are the sort of person to whom strangers 'always seem to tell you their stories', if you find yourself being the go-between when family members have conflicts, or the calm in the eye of the storm at work, if you tend to be a 'problem-solver' or a sympathetic ear for people who have been treated badly, but have never had a really brutal experience with a toxic person, you're high risk because you've just been lucky enough not to encounter one in a close relationship.
Learn all the terms for toxic behaviours in relationships (eg gaslighting, psychological manipulation, stonewalling, blame-shifting, projection etc and exactly what they look/sound like), what 'red flags' are, and promise yourself that no matter how trustworthy someone seems, never invest significant money or time into a personal or employment relationship without a watertight contract.
Investment in this will protect you against decades of potential pain and complications.