There's lots of good stuff already in this post, but I thought I'd throw in a few of my observations. I have been gardening in the same small spot for about 15 years now and I have learned that I need to rotate crops not only in space but in time, as well. I don't have that much room to move things around, and I have found that I can grow the same things (squash, tomatoes, cucumbers, beans) in the same place for a certain number of seasons before pest pressure begins to build. For instance, I grew squash for a couple of years with no issues, then 3 years ago I had an infestation of squash bugs (some of the pictures in this thread brought back bad memories!) so I shut down the squash. No squash until this year and I haven't seen a squash bug yet, despite going overboard on the squash plants. I had hornworms for the first time in a few years last summer, so I don't have any tomatoes this year. We'll see what happens next year. After growing beans and cucumbers in the same spots for a couple of years, I noticed some kind of worm that was getting in the fruits. I swapped their locations and haven't had a real issue yet this year.
Brassicas are a problem for me, though. Some sort of brassica is necessary for a cool weather garden in NC, but the cabbage worms always find them. Time again is an ally, planting at the right time in the fall and the brassicas will begin to grow but then it will get too cold for the worms and I don't have to squish them or spray the bT. I did notice this spring, though, that a mixed planting of some collards with various herbs, mustard and radish had no worms. I plan to investigate/replicate in the future.
I have the constraint/luxury of only growing a few things so I have to find ways to make it an advantage. I am almost tired of zucchini this year and I find myself thinking of fresh tomatoes and homemade salsa already!