Balance:
Lets look at balance first.
This is were your creative intuition should ‘Kick in’.  The exact middle is not a good artistic balance, that is obvious, but what about the less obvious?  Like hills sloping left to right and nothing to balance it from right to left.  In summary all the elements of your composition should be contained with in the confides of the edges of the work so that there is space and detail balancing each other.  It all should create a harmonious one-ness that is inviting to look over.

Contrast:
Sit down and make a list yourself of the variations in the ways we can use contrast.  You may think of more than me.  Now pin this list to your easel and ‘USE IT’.  Never miss an opportunity to place a contrast.  A shadow, gives an opportunity for a highlight, next to it.  A cool colour is a invitation for a warm colour. 

How many examples have you got?  Lets compare: Light and dark, hot and cold, soft detail, hard detail, thin paint, thick paint, moving, static….  I could go on but most would be a variation on this list.