I didn't have too much trouble house training Isabella in the beginning, she caught on to the idea quickly. She does still have several incidents, most usually around the beginning of winter, like she will ask to go out and pee but doesn't want to be out in the cold long enough to poop, so she sneaks off somewhere in the house and does it.
I'd actually decided to try litter box training Ty Lee, with the hope that Bella could learn it too, at least enough to stop going on the floor when it's cold out, but that only lasted a little while. All the dogs I've house broken before, I'd just train to go straight out. Ty Lee started off doing fairly well with the litter box, it actually just had a puppy pad, not litter, but then she decided that was the only place she absolutely will not go.
Usually I have good success if I catch them starting to have an accident somewhere inappropriate, say a sharp "No!" scoop them up and put them down in an approved place and tons of praise when they finish in the right spot, they usually catch on pretty quickly but Ty Lee has the most annoying ability to hold it for insane amounts of time after being interrupted! The whole box thing was quite mutually frustrating us both and I finally gave up after about a day and went back to what I know. She made huge progress in just one day and didn't have any accidents at all yesterday with being taken outside.
