Rationality helping in relationships (here used to mean all interpersonal, not just romance) :
Use "outside view" to figure out how your interactions look to others; not only to the person you are talking to but also to the social web around you.
Focus on the goals, yours and theirs. If these do not match, the relationship is doomed in the long run, romantic or not.
Obviously, the whole list of cognitive biases and how to counter them. When you -know- you are doing something stupid, catching yourself rationalizing it and what not, you learn not to do that stupid thing.
I've often had half-finished LW post ideas and crossed them off for a number of reasons, mostly they were too rough or undeveloped and I didn't feel expert enough. Other people might worry their post would be judged harshly, or feel overwhelmed, or worried about topicality, or they just want some community input before adding it.
So: this is a special sort of open thread. Please post your unfinished ideas and sketches for LW posts here as comments, if you would like constructive critique, assistance and checking from people with more expertise, etc. Just pile them in without worrying too much. Ideas can be as short as a single sentence or as long as a finished post. Both subject and presentation are on topic in replies. Bad ideas should be mined for whatever good can be found in them. Good ideas should be poked with challenges to make them stronger. No being nasty!