As well as what the others have said... . another important one is to be non judgemental.
PwBPD believe that we are always judging them... . so try to stay away from that.Try to stay in the moment too. When your ds is telling you something, listen and validate what he is saying then and there. Try not to go back to all the other times he has said or done this or that and link all past events together. He maybe telling you all lies but dont get caught up on them. go behind them. Validate his feelings behind he is saying.Validation is very important in building trust in any r/s.
Iam also careful not to ask questions that dd18 may think are too intrusive. Typically with my dd they are the why, what, when questions. I also dont give my opinion or advice on anything anymore unless she asks for it, and then Iam carefully not to come across as negative.
Being less judgemental of dd has trickled down to me being less judgemental of others, and better frame of mind which has been benefical to me in the longrun

I hope some of this helps!