One of the best ways to thwart pest and disease problems in your garden is changing up where you plant certain types of vegetables. Vegetables within the same "family" (I'll explain families more below) are often attacked by the same diseases and insects, and if you keep planting members of that family in a particular spot, they're more susceptible to problems.
Vegetable Families Explained
There are nine basic vegetable families: Nightshades
These are your tomatoes, peppers, eggplants, and potatoes. Problems with pests like tomato hornworm or diseases like blight are much more likely if you're planting nightshades in the same place year after year. In addition, these crops are usually fairly heavy feeders, and can deplete nutrients in your soil within a few seasons.
Peas and beans are known as legumes. While they are susceptible to some pests and diseases, the main reason to make sure you're planting these veggies in different spots in your garden is that their roots are able to "fix" nitrogen, thereby increasing the nutrients in your soil. Whichever vegetables you plant after you've planted legumes in your garden benefit from that extra nitrogen boost.
Squashes and Melons
Summer and winter squash, cucumbers, pumpkins, and melons are all part of this family. Susceptible to several pests (including squash bugs, which overwinters in the soil), members of this family are also heavy feeders and will deplete the soil over time.
Brassicas and Salad Greens
Greens such as arugula and mache, as well as broccoli, cauliflower, kale, cabbage, and Brussels sprouts are all members of this family. These crops generally require high amounts of nitrogen, because they put on so much green growth (as opposed to flowers and fruit) over the course of a season.Sunflower FamilyThis is a little confusing, because this family includes sunflowers, Jerusalem artichokes (sunchokes), lettuce, and endive. These crops are light feeders, and grow best when followed by a heavy feeder, such as the brassicas.Carrot FamilyThis family includes carrots, parsley, parsnips, and celery. These plants like lots of organic matter in the soil, but too much nitrogen causes misshapen growth. Best not to plant these in a spot you planted legumes in the previous year.Goosefoot FamilyBeets, swiss chard, and spinach are members of the Goosefoot family. They grow well even in soils with low fertility, so are a good family to plant in places you planted brassicas or nightshades in previous years.Grass FamilyCorn is a member of the grass family. It needs good, fertile soil, so it's a good candidate to plant in a place you grew beans or peas in the previous season.Onion FamilyOnions, garlic, leeks, and scallions are all members of this family. They are good at repelling pests, and require high fertility.How to Rotate Crop Families in Your GardenBasically, the idea is that you don't want to plant members of the same family in the same spot two years in a row. You also want to keep the fertility needs of different families in mind, and try to follow heavy feeders with light feeders. With that in mind, here is a sample crop rotation plan: