First of all, Lilacs do not have to pruned at all, unless you are trying to maintain a smaller size than your variety's natural tendency. There are hundreds of varieties, including a dwarf variety we have in stock, which only grows 3 to 4 high.
That being said, if you are trying maintain a large variety to a more diminutive size, here are some tips:
Taking cuttings to bring inside is actually a great way to prune the shrubs, because it ensures that you are not pruning too late, and thus removing next year's blooms.
Wait until after the bloom to remove suckers from the base of the plant as well.