If you want to extend the longevity of your contained garden, your backbone (central) plant, must be something that will have interest through more than one season.
Evergreen Topiary (conifers and boxwood, for example) will stay green year round and provide an anchor for smaller plants to fill in around, through the seasons. The first picture uses evergreen topiary, paired with simple green plants like grasses and asparagus fern to create a quirky combination.
Another good central plant is Taro (or Elephants Ears). The one in the second picture is a Lime colored variety and can be purchased online as a bulb from Just Fruits and Exotics. But did you know that you can grow this plant from Taro Root that you can buy in the grocery store? Check out the full tutorial. Taro is a tropical plant that will not survive a cold winter, but if you lift the root and bring it inside, not only will be a good indoor plant, but it will thrive again next spring.
Succulents, Cactus and Grasses also make for good multi-season pot fillers. The third container is filled with cactus and silver ponyfoot (Dichondra argentea) and is just one of many eye-catching features in a funky garden tour (worth checking out) in Texas featured by Pam Penick's blog, Digging.
Climbing plants (in this fourth example, white Mandevilla takes center stage) can also make show stopping focal points. These pots were created by Deb Silver in Michigan. In order to use any climbing plant in a container, you will need to provide a support for it to grow on. You can get creative and make a growing frame from a whole variety of materials, but if you are looking for something that will last a gardening lifetime, it is worth checking out the tuteur selection at Detroit Garden Works, they offer these tall curled poles as well as other classically shaped cages for climbers (like Clematis, Roses, Sweet Peas or Honeysuckle) to grow on.
Filling out the container, don't be afraid of skipping the flowers altogether. This Conifer and Black Mondo Grass arrangement is sophisticated and sleek will continue to look good through the fall.
If you need more inspirations, Sara Begg Townsend and Roanne Robbins have a great new book called Continuous Container Gardens that explains how to use less annuals and more perennials to create longer lasting and more exciting potted gardens. It's a great resource for ideas; fully half of the 270 page book is filled with specific 'recipes' for beautiful combinations that, with small substitutions, will last throughout the year.