Just as taxi-drivers have a bump in their brain for finding their way, violinists' brains are supersensitised to the positions of their left hands. These exercises, performed every day, will help the brain to become more aware of the left hand fingers, as well as building up necessary muscles.
You can exercise away from the fiddle, on the bus, or in any idle moments. Do them little and often, and if anything hurts, stop and do it again the following day.
Schumann permanently damaged his hand, possibly from over-exercising (more probably from barbaric medical treatment!): don't make the same mistake.
Step 1: Exercise the Gaps
Your fingers need to be able to stretch. Hold up your hand, fingers together.
Now stretch them as wide apart as you can.
Step 3: Finger Independence 1
Hold up your hand again, fingers together.
Step 4: Finger Independence 2
Now slowly move the first finger away from the other three.
Step 5: Finger Independence 3
Move the second finger to join the first.
Step 6: Finger Independence 4
Move the second finger to join the first. Move the third to join the other two.
Step 7: Finger Independence 5
Now move them back, one at a time (slowly!).
Step 8: Stretch Those Gaps
Now put the finger and thumb of your right hand between two fingers of your left, and stretch them apart. Repeat for each finger in turn.