"Practise sounding out your guide fingers. Release the pressure of the finger and the thumb before shifting. Shift very lightly and listen for the note before touch-down."
Keep the bow moving at all times, release the finger pressure during the shift, shift lightly and 'hover' above the note before placing the finger.
Hovering above the new note allows us to check that we are in the correct spot before committing the finger. As the tempo increases we progress from letting the finger hover above the note to tapping down quickly on the note.