Placing balls in the inner part of a tree is something a lot of folks don't do, many people only decorate the outside limbs of the tree. When you only decorate the outside, you loose seeing a depth or 3D-dimension type of look to your tree. You also loose the inner brightness caused from putting in these simple reflective balls.
What I do is put either inexpensive gold or silver balls(depending on my color theme) you can buy in sets most anywhere. The gold or silver balls give the needed light reflection you need in the darkest part of the tree, the christmas lights reflecting off of the balls gives an inner brightness and warmth to a tree. For a "bling" look I would suggest at least 50 to 75 balls placed in the inner part of a 6.5 ft tree (of course you can use less, this is just how I like to decorate mine.) As you can see from the photos above, try to place the balls in those bare areas and the area right next to the trunk of the tree, this also hides the ugly wrapped pole (in this photo there are only a few showing now for ease in showing you hope to place them, but you can be assured I will be adding much more.)
Go back and take look at the pic again, do you see all the tiny bright light specks being reflected in the ball? Just the few balls pictured have already reflected alot of light.