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.