This glitch is rather easy to use and it benefits both your relationship with Karen and your dog's affection level.
To perform this glitch, simply take your dog to Karen and talk to her many times when you are holding it. It is best to do this when she is inside a building because it takes a while. As you are talking to Karen, you will see her heart level rise.
There are two side effects to this glitch. First, Karen will be at a red heart (or where ever you stop). To marry her, all you need is the kitchen and a blue feather. Also, your dog will like you more. If you feed him and pick him up and run him every day, he might be able to win the first Winter Dog Race.
Just let your cow get sick by not feeding it or leaving it out in the rain and give it the animal medicine. You will probably have to do this three or four times to get large milk. After two times you should have medium milk. And after three times you should have large milk.
NOTE: This trick will not work to get Gold Milk!
Go to the Flower Shop, not in, just in front of it. Look at the first link of fence on the LEFT side, from the front. You should be able to walk through that one link of fence. NOTE: doesn't work on right side.
On festival days, it will always be sunny no matter what the weather report says it will be.
To get infinite music boxes, first go find the treasure map inside the tree in the northeast corner of your farm, then wait until summer and follow the instructions on the map. After yuo dig it up, take it to Rick who will fix it, and then give it to a girl to make her like yoou a significant amount more, and then you can do it again and again and again all summer.