One of the best features of a perennial garden is that they are always changing and evolving from day to day, from week to week, year after year, and from one season seamlessly into the next. This is true whether you garden in zone 3 or zone 7, whether you garden in sun or shade, in the best possible conditions or the most challenging.
Keep in mind that where ever you garden you will likely lose some plants along the way. If it's a particular favorite don't be afraid to try it again but maybe in a different location on your property. It took me three tries before I got a butterfly bush (Buddleja) to grow in my yard. I just kept trying until I found a location that it liked. But sometimes you do have to face the fact that some plants just aren't worth the time, money, and effort to grow and it may be time to try something new. The world of perennials grows more diverse every year with new varieties that may include longer blooms and more colorful foliage. It's always fun to try new plants as well as enjoying your old tried and true favorites.
While planning or adding to your garden there may be some design rules you do or do not like to follow. Just remember it's your garden and it's for your enjoyment. For instance I have never followed the rule of buying plants in odd numbers of 3-5-7 etc. I like my gardens to have a mix of colors, textures, and heights.
When plants are not blooming it's nice to have a mix of foliage color and texture.
In this photo on the far left is Amsonia blue star, Coreopsis moonbeam, a daylily and a Japanese Iris.
In this you not only see the different leaf textures and colors, but also plants of differing heights add interest to the flower bed.
I love a colorful border.
I like a lot of flowers in a lot of colors. To me my gardens are a thing of beauty. I hope you feel the same about yours.