Another way to practise shifts is to release the finger completely during the shift. Playing an open string in between fingered notes is an excellent way to check that you really have released the finger from the string.
This practice technique also helps you to:
Develop a faster shift
Maintain the left-hand frame
Play the note after the shift with a clear, definite sound
Find the best finger angle, because when tapping a finger down from the air it usually finds a good position, without distortion.
Practice this technique only after your guide-finger slides work reliably....