Lilacs are productive plants which shower you with blooms in the spring and then provide a steady supply of shade throughout the rest of the summer. They also can live for over a hundred years, so correct placement is critical to not only giving your lilac a healthy place where it can grow, but also a permanent location for it to thrive. If your lilac is currently struggling, and no amount of proper pruning seems to be helping, then it may be time to transplant the entire lilac bush to a better area.
Wait to transplant your lilac until just after your lilac blooms in the spring if at all possible. While you may not have the option to transplant then, you may have better success in the spring.
Work a foot away from the base of your lilac and start to dig around it to loosen the soil 18 inches deep. As you dig, try to loosen the root ball from the soil but keep it intact. Work until the root ball is no longer connected to the surrounding soil.
Slip the shovel head underneath the bulk of the root ball and lift the lilac from the ground while someone else pulls the plant up and keeps it from toppling over. If your lilac is a shorter variety (chest height or less) you may be able to remove it by yourself.
Dig a hole in a new location, making the hole six inches larger than the root ball of your lilac both in width and depth. This area needs to have full sun during the day and good drainage that will allow the soil to stay moist without standing water.
Set the lilac into the hole. Center and adjust it as needed to keep it from tipping. Fill in the remaining areas of the hole with compost to hold the lilac steady. Press the compost to firm it around the plant, but don't pack it so tightly the lilac's roots can't grow through it.
Spread a 3-inch deep layer of mulch around the entire base of the lilac to cover over all of the old soil and the new compost. Water the lilac well to saturate the soil and water again weekly to keep the plant hydrated. When new growth appears, you can back off manual watering and rely on rain to take care of your plant, with the exception of drought conditions.