Prune them right after they finish flowering.

Yes, that’s the whole trick. Spring-flowering shrubs like lilacs and azaleas set next year’s flower buds almost immediately after they’re done blooming. If you wait until summer or fall, you’re cutting off next spring’s display.

For lilacs: cut out the dead wood and any suckers from the base. Then thin out a few of the oldest, thickest stems to keep the plant open and airy. No need to shorten everything — just remove whole stems at ground level. You can also deadhead the spent flower clusters if you want, but it’s not critical.

For azaleas: they’re more forgiving. Just snip off the dead flowers (or leave them, fine) and remove any branches that look dead or crossed. If you need to shape it, do it now — not later. Azaleas also bloom on old wood, so same rule applies.

One more thing: don’t prune these in fall. That’s when you’d prune summer-bloomers like butterfly bush or hydrangea. Spring-bloomers get their haircut immediately after they drop the show.

Your future blossoms are counting on you getting this right.