I like to mix in self-seeding annuals to my perennials beds, since most perennials give me at most 3-4 weeks of bloom, often only 2-3. Annuals, on the other hand, bloom their little hearts out all summer long. Ones that reliable re-seed for me are borage, poppies (CA and field), sweet annie, calendula, alysum, and love-in-a-mist. These all mix in well with my perennials - if they get too zealous, I just pull some.
