1.) You can plant crops according to individual vegetable families. (most common technique)
2.) You can plant crops based on the nutritional needs of these plants. (soil fertility)
Tip: For crop rotation to work, the same plant should not be grown within a radius of 10 ft. in successive years from the spot where it was previously planted.
It's kind of like Thanksgiving dinner. You don't seat the same difficult people next to each other year after year. Sure, it would be nice to have a few more Aunt Irene's and Uncle Al's who easily gel with anyone, but still, you got to work with what you have! (The 10 foot rule works really well with squirrely relatives as well, test it and you will see instant results).
Just like people, vegetables have specific families to which they belong. Members of the same family tend to attract the same pests and be more prone to the same diseases. Does blight and flea beetles attacking your tomato and potato crops come to mind? Bleh. In many cases, the offspring of these pests or the bacteria that cause these diseases can be found in the soil. You're giving the pests and diseases the slip by outsmarting them when you unexpectedly change locations.
Nightshade family: Includes tomatoes, potatoes, eggplants, peppers. Heavy feeders; need lots of organic mater; precede with grass family; follow with pea family.
Pea family: Includes beans, peas, legumes, soybeans, fava beans, clover and alfalfa. Adds nitrogen to the soil, especially good to turn over soil at the end of the growing season; rotate with any other plant family.
Grass family: Includes corn, wheat, oats, and rye. Plant to control weeds and improve soil; turn under at end of growing season; plant before gourds or nightshades.
Lily family: Includes onions, garlic, leeks, shallots and chives. Do no plant in soil with lots of organic matter not yet decomposed. Try to rotate with pea family.
Gourd family: Includes melons, cucumbers, pumpkins and squash. Heavy feeders; add organic matter to soil before planting; precede with grass family; follow with pea family.
Mustard family: Includes broccoli, radishes, cauliflower, Brussels sprouts, arugula, cabbage, turnips, bok choy and kale. Heavy feeders; grow in soil with lots of organic matter. Precede with pea family.