I must be on a bowing kick lately because I keep posting about bowing...but here is another little trick I use when teaching bow distribution.
There are times that students need to use less bow, or conserve the amount of bow they are using. Some beginners perform with a very crooked bow because their arm is not yet trained to open at the appropriate angle for a straight bow. When I am teaching fast 16th note passages, students who use too much bow tend to sound very sloppy and I encourage them to use less bow. In each of these cases, you can use this little tip. Clothes pins are very in-expensive and can be clipped right to the stick. Enough of the clip hangs down to prevent students from passing the point on the bow where the clip has been placed. If I want students to say in the middle of the bow, I place the clip toward the upper middle and students are then forced to bow in the right place.
It doesn't take long for students to understand bow distribution and be more aware of the amount of bow needed for various passages.