Yes indeed, I should have asked about pruning. If you prune your lilac in the spring, you will never see a flower, since you would be pruning off the buds.
Do any pruning you need to (they don't need a lot) right after it blooms. By early fall it is setting next year's buds.
Once you do get flowers, it is important to dead head them-- clip off all the flowers once they fade, before it can set seed. Lilac seeds are big and heavy and don't travel far from the mother plant. So in order to reduce competition amongst all the seedlings, in nature lilacs tend to bloom heavily only every other year. The in between years it will produce just a few flowers. Stopping them from setting seed allows you to have a shrub full of blooms every year.