Here is a tip which will go a long way toward refinement of your playing: let go of single accompaniment notes at the precisely designated moment.  I am talking about those notes in the left-hand part, usually quarter notes, which may seem relatively unimportant to piano students.

Case in point: the Little Prelude in C Minor by J.S. Bach BWV 999, illustrated here.  Most of you have probably played it.  Did you ever really try to hold that quarter note for exactly 4 of the 16^th notes and then let go of the key exactly at the same time as the right hand starts the first 16^th of second beat?  If you have, hats off to you.  If you haven’t, I highly recommend it.  Not only will it give a crisp sound and a more palpable presence to the left hand, it will bring in that element of counterpoint without which this Prelude can sound quite monotonous.  The effect is similar to a background of an image suddenly coming into focus and presenting a whole new dimension. 

In this case the new dimension will be your increased mental and listening focus, which makes playing a more engaging experience.  This basic improvement of left-hand accompaniment execution is crucial in keyboard pieces of the Baroque and Classical periods where precision is key.  If you go over your basic Haydn, Mozart, Beethoven and Bach pieces (just to name the obvious) you will notice many opportunities to implement this tip and make your renditions sound more polished.  Hope this helps and Happy Practicing!