Disease Prevention and Control:
Many fungal, viral, and bacterial diseases in vegetable gardening stay in the ground for a while, sometimes several years, and are often associated with a certain plant family. There are also parasitic nematodes that can destroy a garden in no time flat. Fusarium and verticillium wilt can be particularly devastating. Rotating the families these diseases prey most upon can prevent an outbreak
There are many bugs that attack specific plant families. Let's say you plant a brassica in the fall. Putting another brassica there in the spring invites those insects that like to eat things such as cabbage and broccoli. But if you put beans or corn, you won't have to worry about that particular bug.
Different families like different things. Legumes are nitrogen-fixers, meaning they put nitrogen in the soil. Conversely, they need a lot of phosphorus for that reason. So, you would plant a phosphorus-producing plant before legumes and a nitrogen-loving plant after. Nightshade plants like tomatoes and peppers are heavy feeders and need a lot of nutrients, so planting legumes before them gives them the stuff they want. Root vegetables are actually light feeders as far it goes, so putting them after legumes would be exactly what you do not want to do. The idea is to try and keep everything getting what it wants naturally, without always having to resort to fertilizers and chemicals to balance the soil.
Allium - onions, garlic, shallots
Chenopodiaceae - beets, spinach, Swiss chard
Brassicaceae - broccoli, cabbage, kale, radishes, turnips
Cucurbitacae - cucumbers, squash, pumpkins
Gramineae (grains) - wheat, rye, corn
Leguminosae (legumes) - peas, beans, clover, vetch
Solanaceae (nightshade) - tomato, potato, eggplant, peppers
Umbelliferae (root) - carrot, celery, fennel
The Chenos can be used wherever in a rotational plan, so we'll ignore the beets. Plant them wherever after whoever you want. The Alliums also are practically free. All you have to worry about is rotating them with legumes so you replenish all the nutrients they sucked up. Rotating isn't much of a problem because alliums tend to stay in the ground for awhile. Grains help cucurbits and nightshade plants by improving the soil composition and controlling weeds. Rotate the legumes with everybody else whenever you can.
Brassica: plant legumes first and preferably follow with compost to let the soil recover, but you could plant a light feeder such as a root or a cover crop
Cucurbit/Nightshade: plant a grain first and follow with legumes, because they are heavy feeders
Root: plant after heavy feeders such as nightshade/brassica to encourage good root growth and follow with legumes since they like the broken up soil, though you really can plant roots whenever you want.
use alliums, roots, and beets wherever to fill in the blanks
follow the chain of legume - brassica - grain - nightshade/cucurbit - legume - grain - nightshade/cucurbit (repeat)
The seasons matter when choosing vegetables. Some like it hot and some do not. Let's say you planted winter wheat in the fall for a summer harvest. For that place, the cooler spring weather is gone. You don't want to plant a spring nightshade/cucurbit in its place, but a summer loving one such as tomatoes, eggplant, or watermelons. If you plant a fall brassica like broccoli you can follow it up with some onion seeds/sets on Columbus day if you're lucky or beets. Maybe kale. Kale likes it cold. Because of this, seeds won't germinate above a certain temperature. If you are direct-seeding seeds, keep the temperature in mind.
The other thing to consider when choosing a vegetable is how long it actually takes to grow. Not every vegetable, or even breed within a certain vegetable, takes the same amount of time. Some tomatoes take 110 days before harvest. Some take only 55 days. In order to keep things operating efficiently and not have wasted time (or not enough), you need to factor in growing time. Give yourself extra, because not all plants will grow to their optimum date, but have a general idea of the time you need to plant or harvest something. If you are growing a mostly transplanted vegetable such as peppers, you also need to determine when you need to sow the seeds in a flat so the pepper is ready by planting time. I generally recommend graph paper for all this.
For short growth plants like lettuce or radishes (think 30 days or less), you can get several harvests from the same area by planting successively. That means planting seeds every two or three weeks until the end of the vegetable's season. You will always have fresh produce that way. Once the radish season is done, you can plant something else there.
Planning the garden doesn't have to be difficult, but it will require a bit of time on your part. If you want to know exactly where everything is going and when, pull out graph paper and make a diagram of your garden with accurate dimensions. Some plants take up more space than others and you don't want to cram in too much.
Let's say you have three beds. List out the plants you want to grow, how long they take, their family, and what season/temperature they like. (You can get really technical with the temperature and that's how programs identify when to plant things for your area, but that's not necessary.) Go grab a calendar. If your last frost date is April 1st and your first frost date is November 1st, you have 214 days of active growth time on average. But some things grow beyond that time into and through the winter. That's why some things are planted within a certain time frame but in reality would go beyond November 1st, but knowing your last and first frost dates for your area is important.
I hope I haven't lost you. Make note of all those dates for the peas on the calendar, including the harvest dates. It is from those harvest dates you can determine when you are next able to put in something else, like cabbage or broccoli. Continue doing this, making note of dates and if you can do succession planting to make the most of the season, all the way down through summer into the fall. Let's say you want to transplant tomatoes on May 1st. You have to start the seeds (if you are starting seeds) 6-8 weeks before, in March. All this must be accounted for, and trust me, you will be glad you put in the extra effort beforehand.
Many of the things from spring can also be planted in the fall, but this time you are planting before a date, not after it. Let's use kale as an example. It is called the Queen of the winter garden, but even it should be started before it actually freezes, six weeks actually. You can probably easily do succession planting until the first frost, using row covers for a bit of protection, and have kale that will be with you until spring.
Once you have the stuff you want and dates, you can decide where they will go. The peas will be out by the middle of June, the perfect time for corn, chard, or squash. A 70-day corn would be finished by late August, leaving time for carrots, cabbage or broccoli before the first frost. So from one bed, you can get peas, corn, and carrots for the year with no time wasted. In between all of this bigger stuff you can easily throw in lettuce, radishes, or herbs. Quick growing things like radishes can even be used to mark rows for slower growing carrots or parsnips, getting two from one at the same time.
With some planning and forethought, you can create a wonderful rotational garden that will not only provide a ton of good food, but do it better and more efficiently with less work from you.