To get as high a friendship as you want with anyone in town, repeatedly try to give them your pet. If you have the Love Bangle equipped (not necessary for the effect), you will see that every time you talk to a person with a pet in your arms their friendship score will increase one point. If you do this inside and have a lot of patience, you can get someone to as high a score as desired within one day. This method is also useful for unlocking the Channel 7 Harvest Sprite, who will appear the day after you earn at least 100 friendship points with someone. If you do not have the Love Bangle equipped, then just show your pet to someone 100 times.
