Just don't talk about common things (like the weather (i like the weather but i don't like to talk about the weather because i'm not interested what for weather it is today, i'm only interested in extreme weather :D ), no things where everyone talks about it, no unimportant things). Also don't ask difficult questions like (what do you do for work, what do you study, questions about the family, ... ). Don't ask things about (how you feel about ..., what do you do when you're angry). INFP's have a rich inner world, but they don't show it to the outer world and don't want to talk about it.
Find their interests, and talk about it. I have a trick to know someone else. You'll always talk about the person where you talk about it and their interests (don't talk about yourself or your interests).
Be warm, open, gentle, attentive. Show that you're interested in it. They will say you don't have to do something special (but that's just a nature polite reaction). INFP's are ashamed when they are good treated but they like it. BUT important: don't do it in public because they won't like it anymore. So don't propose someone for a marriage in public. Never do it. They don't like it to be embarassed and show their inner world/feelings with everyone else.
Probably interests from an INFP are art, movies (fantasy), tv-shows, books, music, unique things, experiences, travelling, a search for something (the truth, a meaning), philosophing about life, about a lot of things. Nature. Psychology. Uncommon things. You can talk about a lot of things but you have to set the focus on this subjects (i guess).
They are not easy to handle/talk with, but that's why they're so special. And if you persevere, you'll find a rough vulnerable diamond inside him that's worth fighting for.