In my garden, I have an insectary composed of plants that offer beneficial insects a place to live, breed, and eat. Beneficial insects will eat the old nasty buggies that threaten to decimate the garden. And you need some beneficial insects, like bees, to pollinate some veggies. Plus, plants that attract beneficials are often pretty.
My main influences for this subject are Gaia's Garden, by Toby Hemenway and Great Garden Companions, by Sally Cunningham.
The important things to remember are diversity, hardiness, and bloom period. Diversity, because different plants do better in different years and different plants attract different beneficial insects. Hardiness, in the sense of being tough, so you don't have to pay attention to them. And bloom period, because you always want something blooming in the garden to attract the good fellas, so it's important to have plants that have long bloom periods or have different plants that bloom in the spring, summer, or fall.
I prefer perennials and self-seeding annuals because they are easier. These plants, which are supposed to host beneficial insects, are currently scattered throughout my garden: Cilantro, Lemon balm, Chives, Parsley, Marigolds, Borage, Dandelions, Thyme, Rosemary, Mint, and Clover. In fact, Cilantro is a little bit too scattered through my garden. I actually have to fight my way through it to get to the rain barrel.
Since I don't have enough projects planned for the fall, I decided to plant a specific perennial insectary at the end of each garden bed, and also incorporate one in each new garden bed that I plant. My future insectary will include: Yarrow, Echinacea, Liatris, and Coreopsis. These are all plants that do well under drought-like conditions (they are often recommended in xeriscaping books), but tolerate a fair bit of rain, and also have long bloom periods. Here is a website for those of you who don't have a nursery that sells xeriscape plants nearby.
I will keep you updated on my progress and the results of the insectary. Hopefully, it will reduce the need to spray soapy water on my tomatoes :).