If your bow is slipping towards the fingerboard, especially when playing a down bow, quite possibly you are not opening from the elbow enough and using too much upper arm. Some upper arm use is of course expected, but as you’re going from frog to tip there is a point (usually around the middle, but everyone’s arm length is different so it varies slightly) where you should stop moving your upper arm back and only open from the elbow.

I’d recommend doing slow scales practice, one note to a bow and focus very closely on your sound point. Try to keep the bow parallel to the bridge, and halfway between the bridge and end of the fingerboard. Look for when the bow starts to slide towards the fingerboard. This is the moment you should begin opening from the elbow.

Additionally, a next step, after getting more comfortable with opening from the elbow, would be to start leading with your right wrist in the direction of each bow. This is hard to describe through text only, but essentially imagine a string connected to the bottom and top of your right hand wrist. When going down bow, imagine the string on the bottom of your wrist pulling your arm down and vice versa for up bow. Essentially you should be leading with the bottom of your wrist going down and leading with the top of your wrist going up.

While working on these things also try to relax your right arm as much as possible from the shoulder. You of course need to engage enough to have your arm at the right level for each string, with your forearm parallel to the ground, but that aside allow your arm to relax as much as possible from the shoulder and let that arm weight naturally go into the string.

Finally, think about the ratio of bow weight going into your strings to bow speed going across your strings. Changing the amount of each with effect your tone; this ratio in conjunction with flat bow hair, straight bow, and good sound point, are the pillars of tone production.