If you have a young Lilac that you would like to thicken up, prune off all the spent flowers heads. These will turn to seed if left to grow; and the manufacturing of seed requires a lot of energy. To cut them off now will allow this energy to produce more side growth or tip growth, resulting in a bushier shrub.
This is true of most plants. If you can bear to cut the flowers off your newly purchased Tiger lilies or Lion shrubs (Oh my, there are no Lion Shrubs) you will cause the energy that would gone into flower production to strengthen the rest of the plant. This is a common practice when starting fruit trees; they remove all the buds for a couple years in order to establish good branching.
This technique can also be expanded to filling in bare spots as well. Let’s say you’re heavy on one side of the tree or shrub. By pruning some of this out of on the heavy side you will cause all sides to flush out more growth, even the bare spot side. After a few years of selective pruning you will have an evenly branched plant. That is, if the bare spot is not due to being on the shady side of the plant that is up against a building where no light can reach it; then all bets are off.