1) practice everyday - preferably something involving actually playing the violin, although I have done a few where we mainly sang the "twinkle notes" (aaeef#e...), or "twinkle sandwich" (aaee11e), or the one to the boil 'm cabbage that the group class works on....
2) try to keep things enjoyable. - play one of our games (snakes and ladders, fishing, giving a concert to stuffies, play together, or just play a few things - generally his choice.)
3) try not to correct very much - and always try to give a specific compliment. (and compliment first, if making a change.) I try to explain why the change would make a difference to the sound (if I know why....)
4) when he is going to play something, suggest something to concentrate on. (let's see if you can keep the bow on the highway..... - hm, this sounds worse than I think it really sounds - may have to listen to myself to see if it is coming off negative sounding.....)
5) If I'm playing with him - let him be the teacher - he loves that.... and I will purposely make a mistake so he can correct me....
6) try to end always positive.
I think this is the main things I'm doing..... or trying to do. It is from stuff I've read, suggestions from my husband, and from what I've seen works best for my son.
His practices have gone a LOT better since I've tried to do these things.
I have considered bribing him - especially for the lessons - but I'm worried about setting up that precedent, as we are going to be homeschooling too, and his sister is watching what is going on - I am really trying to avoid it. But I do have to admit that it is a currency that kids understand.