One of the big struggles for my violin students (and it was also one of my struggles when I was a young violin student) is keeping the bow parallel to the bridge (perpendicular to the strings). It's easy to demonstrate for students that keeping the bow parallel to the bridge results in a nicer sound; just draw the bow crooked across your strings, and then draw it straight; the scratchy, buzzy sound you can "achieve" with a crooked bow pull is convincing to students.

But even if you've convinced your students that it's important, that doesn't mean they're going to do it. Let's face it: playing the violin is difficult, and the beginning violinist has a pile of things to focus their attention on simultaneously. Until some of those things they're focusing on become automatic, they can't attend to all of them simultaneously. And a crooked bow is often one of the ones that suffers.

I can pick a nice slow piece of music, and tell my students to focus on bowing, but they still get wrapped up in fingering, and forget to pay attention to the bow!

I have an observation, and an exercise, that I've found to be helpful for some of my students.

Observation

It should be pointed out to students that most of the bowing motion happens not in one place, but in two. Often, students think of bowing as an elbow motion. In fact, it is as much a wrist motion as it is an elbow motion. I tell my students that the wrist and the elbow are canceling each other out. To illustrate that, I show them what would happen if I tried to draw the bow, and kept my wrist completely rigid. The bow goes very quickly from pointing inward to pointing outward, and then flying entirely off the violin.

So the elbow motion has a powerful amount of control over the direction the bow is moving. The wrist has to have an equally powerful control, because the wrist has to "undo" the direction change caused by the elbow. The arm needs to be a precision machine in which the wrist and elbow hinge in tandem. Whenever the elbow is changing its angle, the wrist should be changing its angle as well.

I demonstrate this for my students, so they can see that my wrist is in constant motion when my elbow is.

Exercise

Have your student hold the violin and bow, with the bow extended all the way to the tip, as though the student was preparing to start an up-bow. If necessary, shift the direction of the bow for them so it is parallel to the bridge. While the student is standing like that, tell them that you're going to take the violin and bow away from them, but you want them to remain motionless, with the exception that, when you remove the bow, they can close the gap between their thumb and their first two fingers (index and middle). This will result in them forming a sort of "ok" sign as shown below. As you are removing the violin and bow, reiterate that their elbow and wrist should not shift during the process.

Now ask your student to look at their hand. What they will see is slightly dependent on their size and the length of their arm. When I do this process, I see my thumb and forefinger, and a little bit of my middle finger is visible within the circle of my thumb and forefinger.

Once they've taken note of how it looks, they should be able to recreate the position they're holding their arm/hand in.

Now tell them to bend their elbow toward their chest, but tell them that they must keep their hand in the same position - in other words, if they could see a little bit of their middle finger inside the circle of the thumb and forefinger, they need to keep their hand positioned so they can see that middle finger the whole time. Once the elbow is fully bent, it should be straightened again. Repeat this motion over and over.

Encourage them to make that motion very slowly at first, so they can focus on keeping the orientation of the hand steady. They will probably not think it's a very "natural" motion, and they're right; it doesn't feel natural at all! That's the reason why it's so hard for students to get the bow facing in the right direction; that wrist motion can feel somewhat unnatural.

Tell students to practice that exercise repeatedly, without their violin in hand. They should practice it every day, doing it slowly at first, but also trying to increase their speed. The goal here is to make this tandem motion of wrist and elbow part of their muscle memory, so they're doing it without even thinking about it while they're focusing on other things.