Watching how normal people interact and do the same thing.
Decide how you want to be treated instead of thinking up ways of stopping baddies abusing you. Once you have decided how you want to be treated, you are completely intolerant of any deviation from that scenario. Does that seem too harsh?
Healthy people cut out toxic people very quickly when they overstep boundaries, even small boundaries, so it never gets far.
Those of us with abusive backgrounds sometimes agonise over whether and how to cut out people, about how to protect ourselves, whether it is "fair" to cut that person out now, whether we should accept the person's behaviour because they've been through a tough time, how can we help them, how they are good underneath if only we could get them to change a few things, if only they would let us help them etc.
Healthy people don't bother with all that, they just quickly dump the loser (friend, wannabe boyfriend, etc).
The biggest step for me fortunately came in my teenage years: that understanding and accepting are two different things. I can understand why you behave as you do, however, I don't have to accept you treating me badly, I can walk away and never see you again.
There is a little bit of recognising that you are no-one's saviour. You aren't that special. They aren't that broken. He/she is an intelligent adult, even if they've had a hard life, they could choose to behave differently, if they don't that's their choice, your only choice is over how you wish to be treated.
As a rough rule of thumb I'd say it is not your job to fix someone else unless they are under 18 and share half your DNA.