Hefty, Hefty, Hefty

"Visual weight can be defined in two ways.  First, weight refers to the inclination of shapes to float or sink.  Second, weight can refer to the relative imporance of a visual element within design." - Mary Stewart

Much like buying garbage bags, it is a no brainer to consider weight in the composition of a piece of art.  If you're going to have a lot of heavy things in your piece of art, you're going to need to go heavy-duty on the rest of the composition.  If you put a lot of heavy things in a thin garbage bag, it's gonna break, and if you put a lot of heavy things in a painting that has very little support, it's gonna be weird, confusing, and quite possibly unpleasant. :)

So what are some heavy things in a piece of art that will add weight?  Dark colors, extremely large objects, things that are very intricate, or areas that have a lot of little small things.  The way to balance out these heavy things and create a balance is of course to do the opposite with the rest of your composition!  Light color balances out dark color, a lot of extra space balances out a large object, simple and more basic areas will balance a very intricate area, and when there's a lot of little stuff all crowded together you just need to give it a heavy slab of nothing. :)

A good example used by Stewart is Kathryn Frund's Radical Acts:

Radical Acts has many intricate little pieces of paper on the wall, which are making that corner of the piece fairly heavy.  She also has a huge black smear across the diagonal of the wall, which is extremely heavy.  However, she balances both out by giving the rest of the wall a lighter tone of paint.  This piece is still slighty heavy, but the empty space and the heavier space balance each other out in a pleasant way, so essentially, it is balanced out.