How do people keep the interior of villager houses intact? Is the only way by not accepting their requests to do errands, get items/trade, etc? Will this affect friendship? I always try to help them out but this means that their houses look weird.
Don't do deliveries or catch bug/fish for them. It's almost guaranteed they will switch their house. Christmas/Toy Day gifts will also guarantee them to swap out an item in their house. I haven't been able to get Coco to switch out her Christmas gift and it looks terrible in her house. She still hasn't requested to swap it out. ;^;
I only accept their fruit, time capsule, and signature requests.
EDIT: I forgot to mention to put gems in Retail. Villagers can buy them but won't display them in their house. If you sell them furniture or fish/bugs, etc, there will be a chance they'll swap out something in their house.
Out of all of my towns, I have to say it's Muffy. Her decor is one of the reasons why I warmed up to her so fast. It just suits her so well, and nothing looks out of place unlike most of the villagers.