Disinfect your pruning shears. Pour 1 part water and 1 part bleach into a bucket. Set the blades of your pruning shears into the bleach solution for five minutes. Remove the shears from the bleach solution and let them air dry.
Identify branches that are at least 3 years old. Lilacs bloom from new branches, so removing new branches will limit the amount of blooms or stop blooming altogether. Branches at least 3 years old will be hard and woody.
Select two or three branches that are at least 3 years old to remove. Stand back and imagine the shape of the tree without those branches. Make your final decision on which branches would give the tree the best shape when removed.
Wrap the blades of the pruning shears around the branch right under a leaf node. Cut the branch off with a straight cut. Stand back and look at the tree. Decide to remove up to two more branches or leave the tree as it is. Repeat if you want one or two more branches removed.
Place the blades of your pruning shears in the bleach and water solution to disinfect the blades. Let the blades soak for five minutes then remove to air dry. Disinfect the pruning shears after every pruning project to prevent the spread of disease.