You need to go slowly with them.

Pisces are so inclined to just jump into relationships whenever they find someone who even moderately interests them (they really wear their hearts on their sleeves). However, this is what ultimately makes them go through relationships so quickly as well. If you want to win over a Pisces, you need to show them that you're in it for a long haul, that you're willing to take it as slowly as you need to and commit completely. Consistency is sexy to a Pisces. Show them you aren't afraid of it.