Left Hand Development
Wrist posture
Keeping the wrist as straight as possible will prevent damage to tendons, allows a greater grip on the fret board and enables the wrist to be more relaxed, making your playing faster. If you sling your guitar very low then good posture is not possible, it is more stylish but less conventional. Your arm needs freedom to move to allow the right wrist angle and this can be achieved by tightening your strap or sitting right. Try sitting cross-legged and see how much easier it is to position your left hand.
Arm exercise
A lot of the movements of the fingers are controlled in the forearm so it is good to exercise them. Exercising the forearm doesn't mean heavy weights and going to the gym because it is mainly endurance that is needed. Endurance is gained from light activity with lots of repetition, try some curling with small weights and build up slowly.
Strength/endurance exercises
Trills are a good way to build strength and endurance in the hand. Keep to one finger per fret when you do these exercises and try to trill as fast as you can for as long as possible. If you feel a strain you should relax or stop. You will soon notice that relaxing your wrist allows you to play faster for longer.
----Tr~~~-----~~~-----~~~
E---5h6p5---5h7p5---5h8p5-------
B-------------------------------
G-------------------------------
D-------------------------------
A-------------------------------
E-------------------------------
Finger stretches
These may cause pain, which can be prevented by good posture so try finding the most comfortable hand/finger position before starting.
----Tr~~~-----~~~-----~~~~
E---5h8p5---5h9p5---5h10p5------
B-------------------------------
G-------------------------------
D-------------------------------
A-------------------------------
E-------------------------------