I googled and found this information, hopefully it will help
How to Prune Lilacs
For the absolutely perfectly shaped lilac bush, you need to prune them each year. Pruning lilacs is a very important part of growing and caring for Lilacs. While some varieties only grow 4 to 8 feet, others can reach up to 30 feet tall. Most will grow in excess of 10 feet. Pruning will not only help with shape and appearance, but also impact plant health and vigor and the profusion of flowers.
There are right and wrong ways to prune a lilac bush. There is also a right and a wrong time. Most importantly, prune or trim back your bush immediately after they are done blooming. Make sure to remove the spent bloom with your clippers. This will keep the plant from growing seeds and encourage creation of next year's buds. Next year's flower bud develops early, even though you may not see it. I have seen inexperienced gardeners trim the tops several inches back. While it looked a whole lot nicer, they loped off the next year's flowers with one pass of the hedge trimmer. By the way, we do not recommend using hedge trimmers. It gives a too trimmed appearance. Lilacs are not hedges.
Lilac bushes should be pruned and maintained each year for a well shaped and healthy plant. The plants should be full looking, yet not overly bushy. If the plant is too bushy, the inner leaves do not get sun and air circulation, an invitation to plant disease.
Trim larger stems from the center of the bush to increase ventilation. It will also afford more room for newer shoots on the outside of the plant to develop.
Pruning should be done immediately after the flowers have died off. Cut small suckers and shoots at or near ground level, or where it comes out of the main trunk. Leave a few strong and healthy new stalks each year, especially if you are planning to trim back old wood.
Trim back any branches that stick out from the main bush, and is not appealing to you.
Topping the bush is not recommended. A flat top is not an appealing lilac shape to most lilac lovers. A slightly rounded top looks best.
It is okay to clip off old, dead flowers at the base.
In trimming and pruning your bushes, remember, beauty is the eyes of the beholder. If you like a tall bush, let it grow tall, If you prefer a wide bush, encourage shoots that have spread out from the main bush.
Trimming Mature Lilac Bushes
If a lilac bush has become overgrown, or is too large or tall for the area you have allotted it, there are a couple of ways to prune Lilacs.
We recommend the three year plan. A lilac shoot takes about three years before it produces a flower. So plan to eliminate 1/3 of the shrub each year, selecting the oldest stalks. Cut them down to just above the level of the soil. As you do, allow new shoots to grow to replace the old ones. By the end of three years, the entire shrub will have been replaced, and you will not go without lilacs for that period of time. Then, continue the cycle each year.
If life with your overgrown shrub has just become unbearable, remove all old stock and leave just new shoots. This is pretty drastic. And, you will go a couple of years with out lilac flowers. But Lilacs are hardy. As long as there are a few healthy new shoots, they will grow back.
I'd rather be gardening!