Well, we typically just lock them in a small-type room with the litter box. Like a bathroom or something. They need that digging sensation before they go to the bathroom, so they will use the litter box. Unless they are totally stupid (lol) they will use it. We had 3 week old kittens that went straigt for the litterbox and never messed on the floor once. They were "farm kittens too". Anyhow...lock them up for a couple days to be sure they are using it, and then let them into the whole house, but have the box in an easy-to-find place. They will typically seek it out and use it. Oh, and when you put him in the real small area where you first litter train them...put a scratching post of some sort in there too. They need to learn that they ONLY scratch their post, and they ONLY go to the bathroom in their litterbox. A plain old post/2x4 nailed to a peice of plywood works good. They typically sharpen their claws in old peices of wood and trees around the farm anyhow.
The first few times you let them into the entire house area...watch them carefully to see if they try to scratch the furniture. If they do, smack em with a fly swatter or newspaper. Dont be gentle or they will never learn!! They will think its a game. We had a leather couch the last time we had to adapt farm cats to house cats. We did all this and they NEVER scratched the couch!! If you are going somewhere and nobody can watch the cat, lock it up in a safe room once again. They are not really stupid, and as long as you provide them with their own thing to scratch they should be fine. They scratch when they are bored and have nothing else to scratch.
The best mousers are probably going to always be farm cats, or kittens that were raised up to 6-8 weeks on the farm. Their mama teaches them to mouse. I dont think the cute little kitty from the pet shop will ever make much of a mouser. Dont get them too fat and spoiled if you expect them to mouse.
As far as keeping them around...call them when you feed them, and make a habit of them coming to the house two or three times a day just to eat. They will know that is where their food is. Maybe even give them something special so they know its a REALLY good idea to go back to the house!!