I applaud the previous answers, especially about Finger Independence. They are helpful.
I've played guitar for 42 years, and have not always been consistent in my playing.
In the last three or four years, I have become not only consistent, but obsessive about playing. Being very sore in my left hand made me review the anatomy and biomechanics I studied many years ago. I developed some exercises based on the anatomy and function of the hand muscles for therapeutic rehab of them as well as to increase dexterity.
Back when I was a student doctor, I spent an entire year in anatomy lab (three trimesters) with cadavers, and took many hours of various class room hours of anatomy, bio-mechanics and other related courses too numerous to list. I remember wondering how much of all that I was taught would be used by me in practice. I'm going to give you a slightly complex answer to your question and then a couple of very simple exercises to improve your finger function for guitar. You can do even them when you can't play your guitar. They are specific for your fretting hand.
I'm referencing "The Extremities" 4th Edition by John H Warfel http://books.google.com/books/about/The_extremities.html?id=hchqAAAAMAAJ
Lateral means away from the midline of the body.
Medial means toward the midline of the body.
Lateral Hand Muscles
Adductor Policis Tranversus
The Muscles listed ABOVE WORK TOGETHER with the Muscles Listed Immediately Below of the Medial Hand Muscles. (We will look at them as flexors. They are not really called flexors, but it helps to get the concepts I'm going to explain for you to see them as flexors for now. BTW they are called Adductors)
EXAMPLE of HOW the Muscles above and the muscles immediately below Work Together:
Bringing the Thumb and 5th-4th-3rd-2nd-1st Digits TOGETHER
Opponins Quinti Digiti
Flexor Digiti Brevis
Flexor Digiti - Interossei, Lumbricales, Profundus, Superficialis
- The NEXT set of muscles are not so much related to the Lateral Hand Muscles as they are to the Medial Hand Muscles for they are EXTENSORS
Notice the Next Set of Medial Hand Muscles are Muscles that TURN ON the moment that Medial hand muscles listed Immediately above Turn OFF.
The EXTENSOR Medial Hand Muscles are listed below
Extensor Digiti Minimi
Extensor Digiti proprius
Extensor Digitorum Communis
EXAMPLE of HOW the EXTENSOR Muscles Work APART from the flexors:
Bringing the Thumb and 5th-4th-3rd-2nd-1st Digits APART
Bring the Thumb and the pinkie finger Together then Apart slightly
Bring the Thumb and the ring finger Together then Apart slightly
Bring the Thumb and the birdie finger Together then Apart slightly
Bring the Thumb and the index finger Together then Apart slightly
REPEAT for a period of time....And Repeat often
Do the exercise with both palms up and down and at the sides of your body as you walk.
Create your own sequences of finger to to thumb movements to practice. Once you are comfortable with them work at building speed. REMEMBER to extend as well as bring them together.
NOTE- If doing this causes your Medial Hand Muscles to even mildly cramp or hurt, then it is time to take a few days break from playing and work on stretching and healing the hand.
Hold the hands out palms facing up for referencing the "anatomical position" ...
Slightly adduct the thumb in (flex it slighty to approximate how the thumb works the neck of the guitar)
turn the hand palms down to do the exercise
EXTEND the pinkie finger slightly then flex it. Repeat for a period of time
EXTEND the ring finger slightly then flex it. Repeat for a period of time
EXTEND the birdie finger slightly then flex it. Repeat for a period of time
EXTEND the index finger slightly then flex it. Repeat for a period of time
After this becomes easy then make up your own repeating patterns and drill them.
Once you begin to get fatigued with the hands turn facing down, then turn the hands facing up and repeat the exercises.