What in the hedge is a lilac?
Lilacs the genus Syringa are beautiful bushes with purple (lilac is also a shade of purple, you know) or white flowers and slightly elongated, heart-shaped leaves of green or blue-green color. If maintained well, lilacs will form thick hedges with clusters of small flowers with a wonderful scent. If left to its destiny, they will grow stems up to 12 centimeters (7 inches) in diameter with flowers only at the top, if at all, and bluish leaves.
A word of slight caution
Once you have lilacs in your garden, they are there to stay. Nothing stands a beating and a bruising or two like the lilac. In fact, it loves it when you get rough. Pruning your lilacs is very important, and is hard to overdo. The worst thing that can happen here is that you get no flowers for a year or two, but they will be back.
Before introducing lilacs to your garden, consider your water resources. Lilacs require vast amounts of water (although they do dislike soggy soil). If your water comes from a well on your own properties, you may find that you are forced to use less water for your household to prevent the well from going dry, even if you do not give extra water to your lilacs, and if the climate is hot and dry, you will need to do so (and use mulch), so consider your water bill and available resources.
Prevent them from taking over your garden
Lilacs will create vast root systems underground, so you should expect new bushes to pop up anywhere within 15 meters (50 feet) from the actual hedge. The lilac behaves like a weed, and where you do not like it, you should treat it as such. Fight intruders on your lawn with much vigilance. You may also want to establish a line past which you do not want the lilacs to march, and mark this with a line of rocks, railroad cross-ties or similar to prevent your hedge from sneaking up on you.
Also, to prevent this behavior to some degree, prune the blooms as described below. This will also stimulate the lilac to produce more flowers.
Fertilizer
Once established, lilacs generally do not require fertilizer. They handle themselves very well anyway, sometimes almost a little too well.
If fertilizer is needed, use one with a low level of nitrogen (the first number in a fertilizer's three-number composition rating). Too much nitrogen will impair blooming. Also, making sure that the second number in the fertilizer's rating is at least twice as great as the first is important for the same reason. Apply the fertilizer to the base of the plant in late winter/early spring. Come summer, you should see the result in the foliage, and the flowers the summer following that, as the buds are set one year in advance.
Rejuvenating old lilacs
If your lilacs look more like half-dead trees than energetic bushes, wait until late winter (when you can be pretty sure the temperature will not drop too far below the freezing point), and bring out the hedge-cutters and the saw. Cut all thicker stems down to about 30 centimeters (a foot), and save a few thinner ones to start the photosynthesis. Do not expect flowers for at least one and a half year, possibly up to three and a half. The lilacs will rejuvenate and come back with more thin stems and greener leaves, forming a thick hedge.
Pruning
Cut off blooms as soon as they have faded, which will usually be between the end of May and the beginning of July in the northern hemisphere, depending on climate and lilac species. This will stimulate blooming and prevent garden takeover.
For a dense hedge with many stems, make it a practice to prune stems thicker than 3.5 centimeters (1.5 inch) in diameter in late winter to about 30 centimeters (a foot) above ground. This will impair your lilacs potential of carrying flowers slightly, but will keep the hedge green and dense.
The same technique can also be used for medium-sized rejuvenation processes. If flowers are more important than foliage to you, do not prune as described above for a few years (but do prune faded blooms), and when needed, take one third of all thicker stems each year for three years, and watch your lilacs thrive.
Remember that cutting below the second level of leaves will make that stem unable to carry flowers for a year or two to come, and anything beyond pruning faded blooms within a week will decrease blooming.
Enjoying your lilacs
Each species of lilacs has a different scent (all of them pleasant), and it can be enjoyed purely for this reason. Sacrificing flowers and keeping the stems young and dense, the lilac can even be cut to slightly less natural shapes such as box-like hedges. The bushes make excellent hedges, instead of fences or walls for maximum privacy and contribution to the feeling of the garden as a green room.
Kudos to BlueDragon, anthropod and C-Dawg for comments and pointers.