The right way: Prepare the ground before you put a single flower in it. Most plants prefer well-drained ground, where the earth is crumbly and dark. If your dirt is compacted, or gritty, it could be depleted, it could be too close the road (i.e., full of road salt), or it could be not dirt at all (hard-packed clay). You'll need to add some organic material, like compost or chopped-up leaves, folding them into the soil to a like you would with cake batter, until you get something that resembles that rich, dark soil.
If you've goofed: Try digging up plants that are still alive, expanding each hole a bit, and adding organic matter—or replace the bed altogether.