I moved into my house about 2 yrs ago and the previous owner never pruned her liliacs or over pruned it (I am not sure which) and now we have a tree that is very tall and skinny and only leaves and blooms at the top. How and when should I prune it to help thicken it up?
usually after bloom, but if it's not blooming well why not now. but if severely pruned, be prepared to wait 3 yrs for rebloom.
Usually you just remove the thicker, older, less flowering canes from the middle. depending on how bad it is, I'd just do a little pruning each year so you'll still have some bloom and not have to wait a long time.
To add a bit more, the reason you prune right after the bloom is the tree (bush) starts it's new blooms for the next season right after blooming. If you cut late in the season or into the fall, you remove all the new blooms for the following year. The above answer is right about blooms, but I would rather see a fuller and bushier lilac and have a more dense bloom when they finally come in. I DID wait about 3 years to finally see blooms, but it was worth it!