Personally, I'd wait until spring.

Firstly, the flowering buds are already set, so each twig that you snip off might cost you the flowers - why not wait until they bloom and stuff them in a vase?

Secondly, some syringa are alternate flowerers, and I'd rather see if it has an 'on'- or an 'off' year - that might influence how much you prune the plant.

Thirdly, now over the winter you might have a perfect time to assess the situation and find out what you really want: a tree, or a big shrub as a privacy screen ... or get rid of it alltogether. Is this Syringa the home of birds that would next winter sit at your window and look into your warm room with a silent reproach? Is it in a corner of the garden where you can't do a lot anyhow - therefore pruning, fertilizing and combing it would improve the whole corner without yo breaking your back?....

Lastly - personally I like to wait at least a year before I do something drastic to an existing garden. Simply ammending the soil, mulching and setting up compost heaps with soft maintenance for about a year gives you a feeling for a place and a garden... and sometimes there are treasures lurking :)

If you prune it now, you will sacrifice next year's bloom. But on the other hand if it's in bad shape you may not get much bloom anyhow, so It might be good to sacrifice one season for a really good bloom in 07.
It's up to you.
Linda C

I've read that you're supposed to cut back 1/3 of the old growth to the ground at a time, but when I look at the plant there's one biggish trunk that only branches off about 2 feet up. Is that where I'd cut?

I doubt you'll kill it either way- altho you may lop off some unmade blooms. If you wait until spring can take the cuttings and root them in the spring and make more!

The guys have repeatedly accidentally sprayed mine with roundup when they spray the fences and even that hasn't killed it. those old ones are tough.

Now get a new cultivar the 'little miss kim" and it won't stay alive for even one season... even without getting srayed with roundup!

Karen

Funny you should have that experience with 'Miss Kim' - it is considered one of the most reliable small shrubs for midwestern gardens. And it is certainly not new, being a selection that was introduced to this country in 1954 from seeds collected in 1947. It is also a cultivar of Manchurian lilac, which is significantly different in form, habit and flowering than the common lilac.

jea32, I'd wait until after flowering in spring. While I'd not hesitate to do rejuvenation pruning on a typical common lilac now, one that is grown in a tree form (single trunk rather than multiple stems/canes from the ground) should be treated a bit differently. Too severe a pruning at this time of year might well jeopardize its future, specially in a colder climate.