Pisces is the sign of depth and spirituality, which doesn't mean necessarily religious. It means that the Piscean is looking for a soul mate. The Pisces believes, whether or not he or she will say so, believes that there is a special someone destined for him or her.
So if you can immediately upon meeting a Piscean, if you can talk about spiritual things or New Age things, or something karmic, or some book you've read, or something that speaks of a deeper sense that we are in larger cosmos, that will immediately appeal to the Piscean.
Pisces is also the sign of fantasy, and very often a Piscean can get very lost in fantasy. But fantasy is the key to a relationship, to keep it magical, to be romantic, to give the Piscean a sense that this is a lovely, deep, romantic, to use the word again, relationship in which the Piscean is treasured.
Essentially, I think the word is soul mate. To enter into a relationship with a Piscean means that the Pisces will believe sooner or later that you are his or her soul mate.