When I do mixed plantings, I often combine perennials and annuals in one container. The following year, I'm faced with gaps where the annuals were and I have to decide how to fill them. I've shown an example here. In this galvanized tub, I have three perennials: a heuchera and, barely seen, a chocolate eupatorium and a swedish ivy. A lot of empty space remains where I had annuals planted last year. I found a few more plants at the nursery and to see what the arrangement might look like before I commit any further, I place them in the pot before I remove them from their original containers. Once I'm satisfied, I can go ahead and plant. It's always a good idea to sprinkle in some time-release fertilizer like Osmocote in the planting holes before adding the new members of the container.
In this container, I added coleus "Dipt in Wine" (yes, I bought it just for the name), creeping wire vine, sedum "Vera Jameson", and origanum "Kent Beauty".
Bookshelf: Container Gardening Books