For the lilac: after it blooms, cut it down. You will lose next year's bloom, but after that it will bloom.

While you can cut both of them back severely, they will not remain at that height for long. Even the rejuvenation pruning Ken refers to is just a temporary fix at best although a bit slower process overall.

Appreciate the input, thanks!

For multi-stemmed shrubs-those that exit the ground in multiple stems-you can always coppice, ie. cut the thing right down to the ground, and it will burst forth with lots of new stems. Then, like gardengal says, if it happens to be shrub like lilac where the bloom buds are set the year prior, you will lose one season's worth of blooms but it can be well worth it. Otherwise, the technique which I call renewal pruning-and which I think might be Ken's rejuvenation-I'm not even sure-will work well. And you can do this more or less indefinitely.

Thanks, everyone.