1) Plan for your space.
Whether you use graph paper, a computer, or the back of an envelope, what goes where is the perennial problem. Plants will affect and shade each other: Lettuce likes to be on the east side of something tall, so it gets morning sun but afternoon shade. Peppers like hot sun overhead but some shade on the sides to avoid sunscald. (We like to surround them with basil.) Onions can't stand any shading--they want sun on the shoulders of the bulbs to ripen them.
There are trade-offs to make--Do I want a whole winter's worth of squash and dry beans to store, or fresh sweet corn every day in August??
2) Plan how far ahead to sow indoors.
If you sow squash indoors at the same time that you sow tomatoes and peppers, you will have pot-bound, sickly squash plants at planting time. You will need to start seeds in waves: Tomatoes and peppers 4-8 weeks before last frost, squash and melons 2-4 weeks. Find your last frost date HERE
3) Plan for harvest all season and all year.
There's nothing worse that getting to the height of summer and finding that your sweet corn is already gone, your lettuce has bolted, and your green beans are petering out. Crops that have a short harvest window like corn and bush beans should be sown every two weeks, a few at a time, if you want a continuous supply. Leave space for planting later varieties. Once the original plants have petered out, it is too late for new sowings to mature. And weeds will just fill the space if you harvest without replanting something. Even tomatoes need a later wave of plants if you use determinate types.
Account for early spring, late spring, summer, fall, and winter slots. You can figure a crop will use its space for two or three of those slots (some salad greens excepted.) For example, peas, broccoli, and lettuce could be sown in bed A in early spring. They won't all be finished in late spring, when it's time to plant summer tomatoes and beans--those will need to go in bed B.
In summer, the peas in Bed A will be done--just in time for sowing with kale and cabbage. Those will stand through winter.
The tomatoes in bed B will also use 2-3 slots, going through summer to fall, and giving way to a cover crop of rye and vetch that will be in place for fall and winter.
4) Plan for fresh eating and for preserving.
It's easy to overplant things that have to be eaten fresh, like zucchini. Remember to allot more space for easily-stored crops like winter squash, flour corn, or dry beans than for things you will only use fresh.
If you're canning tomatoes, freezing green beans, drying zucchini, or making sauerkraut, you will need more space for those crops than if you were just using tomatoes, zucchini, green beans, and cabbage fresh. You may want to choose a special variety, too. I use paste tomatoes for salsa and canning, so I plant a lot of those, and a smaller number of favorites for fresh eating. Pole beans give a bigger total harvest over the course of the summer, but if you want to also can or freeze green beans, you might plant a bush variety that will give you a lot at once to get the job over with.
5) Plan to prevent disease.
It can be hard to stop putting spinach in that perfect spot by the path and tomatoes in back where they make a screen. But once you get spinach wilt in your soil, it takes 7-10 years for the soil to cleanse itself. Different organisms target tomatoes. So switch them. Often home gardeners don't recognize a specific disease--they just notice their garden is no longer so productive. It's really important to rotate the plant families from year to year. Here is a guide to plant families and how to rotate them.
6) Plan for fertility.
Legumes pull nitrogen from the air into the soil, so they are often grown either along with a heavy feeder or immediately after. Grains make lots of carbon in their stalks and leaves, giving you more and better compost. And cover crops benefit the soil in more ways than we can list here. They are important enough that it is worth figuring out ways to work them into your garden year. For summer cover crops, a good rule of thumb is to plant buckwheat whenever you have an open spot for a few weeks.
7) Plan for seed saving.
We are often contacted in the fall by customers who want to know how to save seed. We often have to tell them that they are not going to have usable seed--they didn't plan for needed isolation. So, if you have favorites you'd like to save, now's the time to plan. Is it a variety that will cross or is it self-fertile? Do you have more than one variety of that species? Do you have enough distance between them? Can you separate varieties by sowing time instead of distance? Can you plant early enough for the seed to fully mature?
Many flowers and herbs are easy. Tomatoes, peppers, lettuce, beans, and peas are very do-able at home. Squash, the cabbage family, spinach, amaranth, and pumpkins may need special measures. It also depends on your site and climate. Luckily, there are excellent and very readable books. I recommend The Seed Garden by Michaela Colley and Jared Zystro, or Seed to Seed by Suzanne Ashworth. You can download good information here: https://seedalliance.org/all-publications/?fwp_publication_category=how-to-guides
8) Plan for your climate--and some surprises.
Gardeners often love a challenge, but for reliable crops, plant varieties that are adapted to your climate. In short-season areas, choose early types. In hot, wet places, disease-resistance is a priority. Climate will also affect spacing--very wet areas may need wider plant spacing so air can circulate. In hot-summer areas, most leaf crops will appreciate shade in the afternoon. Put them on the east side of a house, a tall row of corn, or a tree, where they will get morning sun and afternoon shade.
Once you have settled on the varieties you want, include some fall-backs in case the season is hotter or colder than you expect. I can nearly always ripen the big luscious heirloom tomatoes. But I always plant a super-cold-hardy type like "Stupice" just in case. If you have cabbage, carrots, and chard as well as tomatoes and corn, you will still have something from the garden if the summer is cold.