Be clear that you want them around (they are super-sensitive to busyness, potential rejection, abstraction, impatience, annoyance etc)
Consistent character no matter who you are with. (especially in how you treat other people and how authentically yourself you are)
Don't debate for sport. Only do it if you truly want a discussion. INFJs will only show their interests, passions, opinions and beliefs to those who really want to know and who are respectful of the privilege they are being given. Otherwise you don't get to know any more or you need to apologize and proceed carefully. This also goes for mocking anything that the INFJ holds near and dear - ideas, places, people, work. They don't bother spending time with too many things they don't really care about.
Care passionately about something and share what it is and why you are so excited about it.
Share your thoughts/thought processes. INFJs don't like being left to guess and usually assume the worst, making them rather emotional (and unattractive to NTs). They also do better if you resolve conflict rather than leaving it be.
Make them feel comfortable before they are thrown into a situation. Two of my boyfriends in the past won me over by teaching me something and making me feel confident at it.
Pay attention to details and remember them. One boyfriend surprised me with the Charlie Brown Christmas movie (a yearly tradition for me) and my favorite flavour of ice cream months after I had mentioned them.
Allow INFJ to do things for you and show your appreciation for their thoughtfulness. They almost can't not give, but they also like people to notice that they have tried really hard.
Tell them why you like them, were interested in becoming friends (dating,whatever). They like knowing how other people view them and what makes them stand out.
Curiosity is attractive to INFJs. So is people who are good at fixing things and the practical aspects of life, while still being able to talk as an N.
Criticism is something that you need to do very carefully. It often is taken personally even if meant very impersonally by an NT as an observation or statement.
Find ways to show that you are thinking of them. It may seem like coddling a bit when they know how you feel, but it will pay off. Notes, a wink or a smile across a crowded room, looking for their eyes when you walk into a roomful of people, making traditions for special occasions and so on will all pay great dividends. You people are pretty creative.
If you need to go into a mental cave to think, say good bye before you leave (preferably not leaving while the INFJ is talking) and if you know, mention when you may be back. Otherwise you risk provoking an emotional response.