Hmm, in my admittedly limited experience you must be driven and fascinating on some level to catch an ENFP guy's attention. They are looking for someone with depth, strength, and wisdom - be prepared for a lot of long, soulful talks. But since they're always bouncing all over the place looking for new experiences, you should be pretty gung-ho and adventurous yourself. Trying to be shy and demure won't get you anywhere. Neither will being fake or frivolous.
As to how to keep one - show him a strength of character and passion that he doesn't have in himself. He'll be drawn to your depth and want to explore, understand, and emulate you. At the same time, make sure never to tie him down and enthusiastically encourage him on his adventures, and either go along or go on your own adventures and tell him all about them later.
Of course, I have to put in a caveat that I've never been in a relationship with an ENFP male. But I've been asked to be in one, so maybe that counts for something.
