Divide and hour up into 4 parts of 15 minutes each. For the first 15 minutes, improvise (preferably exploring some musical and creative phrasing, and using a variety of scales, arpeggio, and intervalic ideas) without using your 4th finger of the left hand (or right hand for left handed people). For the following 15 minutes, do not use your 3rd finger at all, just fingers 1,2 and 4. For the next 15 minutes, just use fingers 1,3, and 4, and for the final 15 minutes, just use fingers 2,3 and 4.
This is a variation on the "restriction as the basis for development" principle which I outlined in an earlier blog, where only the inner 4 strings are used to perform exercises. This time the exercise is designed to break up the patterns that fingers get so used to when practicing scales and other technical exercises across the fingerboard. It can be frustrating, but it's worthwhile because once this exercise has been completed, returning to all 4 fingers on the fingerboard opens up a wealth of potential!