The goal of this simple series of exercises is to prepare the left hand for the technical difficulties that may arise during a rehearsal, practice session, or show. As opposed to the right hand, the left hand usually has to perform really complex finger independence feats, as well as having a remarkable elasticity.
In order to prepare the left hand, it is convenient to have a routine that deals with stretches in a progressive fashion. Play the following melodies, taking care that your left hand fingers rest where they have just pressed the strings and move the minimum possible when the adjacent finger is playing. This will help you maintain the shapes and get the left hand working.
You may play the notes with alternate picking, down-picking only, or your fingers. You can play the melodies note per note or just strum each group of four notes as a chord. It is also recommended to start the exercise on different strings, and in different positions of the neck (taking particular care to avoid the lower positions, as these demand extreme stretches and may hurt your hand if you play them carelessly). The example below shows a variation of the exercise beginning on the sixth fret, and a second one on the eight fret and ranging from the second to the fifth string.
Once you have done this a number of times, you can proceed to adding some stretches.
And finally, try the hardest stretches, between the ring and middle fingers.
Once you've completed this routine, you are encouraged to try combinations of all these exercises, also alternating between strumming and picking each note individually.