While plants with interesting foliage are very trendy these days, it is also an important design principle in perennial beds to have varied foliage. For perennials that only bloom for weeks rather than all summer, you need to have contrast in shape/texture/colour of leaves if you want the garden to look interesting all summer long.
Here's an area in my new back garden that I'm particularly happy with. Remember that a lot of these plants are babies and are going to be twice as big by the end of the summer or next year, so there won't always be so much empty space.
The plants are:
sea thrift and moss phlox for green low-growing, needle-like foliage
sedum for variegated ground cover
iris germanica for flat, sword-shaped leaves
helictotrichon sempervirens (blue oat grass for fine, blue grassy foliage)
castor bean 'impala' for broad, flat reddish leaves (this is the only annual and I started them from seed)
[Update: here's the same area 3 weeks later with the sea thrift and moss phlox starting to bloom]