When it comes to seamless bow changes...

Smooth bow changes in the middle of the bow, or the tip, or the frog aren't created in the same way. It's about listening and awareness of bow balance and angle. So, practice, practice, practice making your changes as smooth as butter at all parts of the bow. Start with notes on the same string, then from one string to the next, then with shifts. Work on what is hardest for you.