There cannot be an universal answer, as there are many ways to express true love. INFP-wise, I would think:
-Trying to get to know the most "real" you as possible, trying to figure you out, what makes you tick, what makes you happy or sad.
-Accepting the real you without conditions. Unconditional acceptance is one of the most sincere ways to show affection, although this probably goes for all types as well. But INFPs are generally very good at accepting you as-is, not wanting you to change to meet what they wish you were-for they wish you are who you REALLY are.
As far as THIS INFP is concerned (and in addition to the above):
-Words of reaffirmation-I would let you know how awesome and special you are, and will always lift your spirits. I want to see you happy and confident about yourself, I want you to learn that you are a wonderful, special person that deserve to be the happiest individual in the world.
-Self-sacrifice: I will sacrifice my time/money/even a few of the things I like for you. The reason I say "few" is not to limit my love, but because I shall do so without never stopping to love myself. Love puts the significant other first, but truly, one should never stop loving oneself either, because we are as much deserving of our own love as our significant other. In practice, when I am in love, I gladly sacrifice myself for the person I love, although I do so with much more wisdom now than I used to a few years back.

Love can't be selfish to me-it must be selflessness incarnate (of course, paired with self-love and self-respect.)
-Unique gifts (regardless price-they can be expensive or affordable), used mainly as symbols of love, that note that I've been paying attention to what you say/express. I will find out what makes you happy and give you gifts accordingly. Gifts that speak about our conversations. Gifts that honor what your ideals are. I love romantic gifts like that, and don't like gift giving to impress, but rather to express how I deeply care about you and your values.
-I will seek your happiness rather than something in exchange from you. I won't look after you for my own pleasure, but rather will lookout for whatever makes you happy. Of course, as similarly stated above, I will not neglect that I need to be happy as well, but I will make sure you know that I love you without strings attached, and don't plan to ever use you, gain something from you, or betray your trust.
Thinking about it, I used to be not so clear in the past, and did many of the above things without ever expressing that I loved the person. I've changed since, and will let her know sooner rather than later, as it's only fair for each of us to know where we are standing. Some of these symbols of love of mine were not taken as such in the past, and were constituted to be no more than friendly gestures, even though I was really loving the person. So I do recommend for INFPs to, whenever possible, make your intentions clearer-you won't be the less romantic due to it.
