Do not be afraid to prune! It keeps plants healthy and looking their best and is simpler than you may think. Hide cuts by making them where they are surrounded by remaining foliage. Remember that the last leaf bud or twig will determine the direction of growth, so select one that will not grow in an undesired direction. Here are a few more rules of thumb.
If a limb is too low, taking a bit off of the end may remove enough weight to heighten it. Limbs that are crossing, rubbing against each other or growing towards the center are not a good thing. Obviously remove any diseased, dying or dead branches. Stand back and look at the tree from different angles to get perspective of what looks good and what specifically needs removing.
For most, it is best to prune right after the blooms fade or you will be removing the following year's blooms. Prune back what is out of shape or oversized. It is typically recommended to prune out a third of the oldest wood at the base. For many shrubs, if there is a lot of old, unproductive wood or it is out of shape, you can cut the whole thing back hard, but there is a chance it will not bloom the following year. That is what I usually do in my garden. Soon new growth fills in beautiful and lush. If your Lilac (or some other flowering shrub) blooms are six feet high and everything lower is old, dead wood, they have not been properly pruned.
Have you ever noticed how hedges tend to have a lot of dead at the bottom? That is usually because they are pruned backwards. When they are pruned so that they angle out at the top, it shades the bottom and it dies out. Hedges should be pruned in a slightly pyramid shape so that the bottom foliage receives sun, too. Depending upon what the hedge is of, if it is out of shape, it can be pruned back hard for fresh, new growth.
While some will not send up new growth and need what they have to store energy for the following year, many will send up new growth when cut back and some will even bloom again.
Many roses bloom on new wood, so prune in the spring to remove old wood and encourage growth. I recommend pruning back fairly hard (be careful not to cut past the graft on grafted roses) at least every few years. You can also prune as you deadhead. Roses that bloom on old wood should be pruned as shrubs.