There are a number of different approaches to gardening and I subscribe to many of them. In some areas I strive for simplicity and intentionally grow individual plants, well spaced, to accent there presence; my fruit trees fall into this category as they become focal points in various parts of the landscape. In other beds I hope for a mass effect with an explosion of different colors, textures, shapes, and sizes. In some gardens, particularly new ones like mine, it takes awhile before that goal to be reached.
At the peak of the season before plants begin their decline in fall, take the time to analyze your garden beds to determine if they're meeting your expectations. Take pictures. View them from all angles. Be critical of your own design. Identify what you like. Make decisions about what you want to change.
I'm using this time to note specific gaps and holes in my beds that need to be filled with new plants. When I first planted my beds last year I had a plan and placed plants accordingly. Not everything grew as expected. Some died during the harsh winter. Some turned out better than anticipated. Overall, I'm pleased with the results, but there is always room for improvement.
In one bed friends helped me plant dozens of daylilies. Only a portion have made it this far, due primarily to poor stock from the online source and poor storage methods before planting, by me. Most of the plants that survived are doing well, but there aren't as many as I'd like. An analysis of the site shows gaps that I want filled.
Across the yard I have two daisy clumps that are doing amazingly well. They're a Shasta Daisy variety that overwintered fine while the many echinacea and snapdragons that were planted in the same bed last year failed to return. Obviously the daisies like that spot so I'll reward them by expanding their presence. I'm also thinking of adding other flowers from the same family like Aster, Chrysanthemum, and Calendula; I have a few mums and marigolds in other areas and may move them there.
Gardening is a process. Planting according to a plan and then never returning to that plan may work for some people, but not for me and many other gardeners. While letting the flowers in a bed determine their own destiny may be an intentional option for now, without oversight and continual analysis a few plants can overrun and choke out the others, undermining overall intentions. I still like order and want my beds to fulfill certain visions I have for them.
Once you've identified an area that deserves new plants determine the best time for action. Not all plants should be planted in the fall after you've decided to fill in spaces. Spring may be a better time. Sure you have to wait six months or so before acting, but you give the specific plants a better chance at survival. Marking the locations for new plants becomes important.
Whether planting right after your analysis or waiting until spring, use the photos you took (you did take photos didn't you?) as a template. Print the photo and actually draw the new plant on it. It helps to give perspective of how a new plant will look and acts as a treasure map if you have to wait through winter before planting. In spring you pull out the photo, observe where the established plants are in the photo, look for their new green growth in the garden bed, and put new plants in the appropriate spot marked on your map.
Whether changing a plan by adding different plants, expanding a bed with new plants, multiplying the plants already in place, or letting things spread out naturally, by pausing to determine the best course of action you will ultimately improve how your garden looks and how you feel about it. It's often difficult to determine how a garden bed will look in the future and we often choose the wrong time to try and figure that out. When the other plants have reached their peak, it's usually a good time. So look for the gaps and bare spots and decide if you want to do something.