It's pretty easy to win over any of the girls. You just have to give Muffy almost any flower, or you can give her the Pretty Ore. Also, she only likes one fish--the Big Huechep. To Nami give her artifacts and in the fall give her trick blue flowers. Give Celia flowers and she will love you anyday.

Give your girl one of these things each day and she will be falling for you in no time. If you do have trouble getting Nami to fall for you, just propose to the other girls and she will get jealous, and end up liking you more.