Just as a painter needs to add tonal variety to a painting, so too must a musician add dynamic variety to a piece of music, it would be very dull otherwise.
In this video I look at a simple device to add these dynamics. It is not just about playing a crescendo or diminuendo though, it's also about tension and release.
The video doesn't tell the whole story... it is a very basic introduction to dynamics. Playing louder as the notes climb in pitch can be useful and, for example bring scales to life, but one extra thing I should mention is that when the volume increases as the notes climb, it can be very pleasing to the ear to hold back the volume on the highest note.
A simple exercise to develop your use of dynamics would be to play i and m alternating on the open top string, start softly and gradually build the volume and then relax it. The next stage would be to apply this to a scale and then to a phrase from a piece of music you are working on.
Here's a handy hint that minimises left hand finger movements, two for the price of one. Using this technique helps the left hand become more efficient resulting in smoother playing.
A huge contributing factor to smooth playing is preparing the LH fingers early... Perhaps difficult to adjust to at first (focusing on fingers that aren't fretting), but if you scribble reminders on the music, eventually control will become more straightforward.
Time for another exercise.
This one is pretty essential and it breaks down the movements of the fingers as they strike the strings. As with all the exercises, slow and methodical repetition will speed up progress.