When your Spring shrubs are looking like this, and you want next year's flowers to be at eye level and not sky level, especially when those flowers are fragrant (Lilac, Philadelphus, etc) Now is the time to get out the pruners.
The traditional way is to take a third of the old stems, and cut them back to near the base. This gives a more open, natural look,
or you can cut them back to a more formal looking bush with a hedge trimmer or garden shears. New growth will soon hide any bare spots and both methods will produce a glorious display of flowers come next Spring.
For less compact shrubs, I prefer dead heading and cutting back old stems.
If the shrub, or small tree has showy berries, then simply shorten any branches that are getting to high, or wide.
During this yearly pruning, you may come across unwanted guests.
Like these tent caterpillars, remove the branch, making sure to get all the caterpillars and burn.
Sometimes a shrub maybe rather wild and the flowers insignificant, but wonderfully fragrant, a must for keeping at nose height.
Occasionally a shrub will have wonderful flowers, vibrant seed heads and colourful leaves.
Prune as little as possible and give it room
This stunning Philadelphus, planted to hide the utility pole, when in flower has this rather relaxed, draping habit, but when pruned, becomes upright, allowing the underplantings to grow and flower.