I was shopping at Dollar Tree (surprisingly some great teacher gems are here). I came across these stickers (3 colors: red, blue, and yellow). It came with 315 in a pack. So I picked up a couple packs for my orchestra. I put the three stickers in three spots of their bows; one at the balance point, one in the middle and one near the tip. This worked great with my students!
Not only was this a great visual for my students to see where to place their bow when we are rehearsing studies, warm-ups, and repertoire, but for me to do a quick look and say quickly: "set at the red" or "set at the yellow". Even though I reinforce the terms lower half of bow, at the frog, set in the middle etc, this works great! I am able to move quickly in class and while we are playing I can say quickly, set at the blue. Now there are other color combinations you can buy, but this was what was available for me at the time.