Hi John,

Thank you for contacting the helpline. As a general rule, you ideally want to prune your lilacs right after the blooms have faded. They will begin to produce new buds in early summer so waiting to prune later in the season means you will risk cutting off next year's blooms. 

There are two methods of lilac pruning: renewal pruning - where you prune out not more than 1/3 of the branches at a time to maintain the health and vigor of the plant. Remove some of the older stems and branches, allowing more airflow and light and encourage new growth on younger stems. Or rejuvenation pruning - where you cut the whole tree back about 6" to 8" from the ground. This will encourage new shoots and growth but will not bloom for a couple years. 

For bushes that have become too tall but are healthy, you could cut back half the height this season and the other half next year. You'll want to prune just above a healthy side branch keeping an eye on the overall shape of the plant. Here's a little more info on lilac pruning:

Pruning Lilacs

The arborvitae however will not grow back from major rejuvenation pruning. Coniferous plants form buds only either at the tips or along the green growth part of the branches. If you prune a conifer branch into the old wood beyond the green growth, that branch will not put out new growth or grow. This can leave a blank spot in the side or top of the plant. You will have to take special care in pruning to retain the desired shape and look of the tree. Here's some great information on that topic:

Pruning Evergreens

Feel free to reach back out if you have more questions. I hope this helps!