It's easy to get into a relationship with the idea that you'll be able to encourage your partner to change -- just a little -- to be closer to your ideal. But that idea isn't usually realistic or very fair to you or your partner. On top of that, the little projects you have in mind, like getting your partner to get a haircut, become a fan of your favorite band or stop wearing those awful shirts, are an obstacle to intimacy.
Acceptance goes hand-in-hand with encouraging another person to share the most intimate details of his or her self: hopes, dreams, goals, feelings and personal history. Your partner needs to feel confident that you'll accept and appreciate all those thoughts and feelings, not dismiss or make fun of them. And the same goes in reverse: Your partner should accept your thoughts and feelings, too.
Of course, there are times when change is necessary and important, like if you want to become thriftier so you can save up enough to buy a house, or if you or your partner's habits are causing health problems. But even then, your work should be based on trust and acceptance rather than criticism and judgment.