Contrary to popular belief, the guitar can be quite a physically-demanding instrument to play. Other than the inevitable blisters and callouses, playing the guitar can cause considerable strain and injuries to your hands, wrists and forearms when proper technique is not adhered to. To help avoid these pitfalls, a simple warm-up routine before starting to play or practice (especially when performing technically demanding pieces) is in order.
First, it is recommended that you do some light stretching on your fretting hand. GENTLY pull your fingers back towards your elbow to loosen up your wrists. Hold this position for about 10 seconds or so. Next, do the same thing with each individual finger, stretching out the tendons and metatarsals in your hand. Conduct the same stretching with your right hand as well.
Now for some actual guitar-playing exercises! For the left hand, a personal favorite of mine is some legato technique. Legato is Italian for “fluid, tied-together movement”. For guitar players, this involves minimal picking with your right hand, and mostly (if not all) hammer-ons and pull-offs with your left. “Three-note-per-string” scales work great with Legato technique. As for the right hand, I usually like to warm up with any kind of string-skipping exercises. These can be completely mechanical, but they work best when trying to play something musical with the exercise.
When warming up and conducting these exercises, it is always recommended that you use a metronome when playing them. A metronome is crucial for not only learning to stay in time with whatever you are playing, but also for tracking your progress as you advance as a player. Sticking to these guidelines and techniques can help you avoid injury. Happy practicing!