When they ask for fruit, give them perfect fruit if you can~ Big boost.
Write them letters.
Lists of their favorite gifts are on the web. Give them their favorites.
Do their favors whenever possible. If you can't do it, still say you will. You get SOME affection from it.
Talk to them every day.
Don't miss their birthday and give them a favorite gift on said birthday~
Above all, just don't let them feel ignored. Friendship's pretty easy in Animal Crossing~