String crossings should be smooth and relaxed, without any accents. They ought to be performed with minimal arm motion, and the player should anticipate the change with the bowarm. Also, the player must be careful to put the same weight on all strings.
Slurred string crossings should be seamless, with a reduced geometry of the bowarm.
Two good string crossing exercises are :
To play all four strings in the same bow, feeling the entire arm motion and the different levels of bowarm
To alternate between two strings, using rotation of the forearm