Here's a little ditty I worked up, one day at the ranch, for a student who confessed she had a left-hand death grip on her violin.
More often than not, the "grab-it habit" is tied to the "pincer reflex" between the thumb and first finger. Starting off your practice with an exercise that avoids the first finger can help soak your hand in the feeling of "anti-grabbity." Then, you'll be able to keep that relaxed feeling in mind throughout your practice.
While the tune above is perfect for more experienced students, here's another version just right for newer students. Start your warm-up with this no-first-finger tune to help soak your hand in the feeling of "anti-grabbity."