In many routine activities (such as turning a doorknob or picking up a spoon) your fingers and thumb grasp together. This tendency often continues when placing your hand around the neck of a guitar, which leads to tension in the hand and other fingers. Your left thumb should normally rest gently against the back of the neck without squeezing. There are times (such as barring) when the thumb must apply a little pressure, but normally the fingers should do their own work, assisted by the lower arm as needed. Imagine a large rubber band from the back of your elbow to a wall behind you, gently pulling your arm back and helping to keep the fingers firmly against the fingerboard. It's also important to keep the left wrist loose as there is sometimes a tendency to bend the wrist inward towards the fingerboard to help increase finger pressure.