Once we had all of our garden beds built, we had to decide how to lay out the plants within the beds. We try to spread out our vegetables so we have a little bit of everything everywhere. We also rotate our vegetables and don't plant them in the same location year after year.
For example, last year we had sweet potatoes in this portion of one bed:
So this year we planted lettuce there:
Where last year there were tomatoes, this year we also planted lettuce. (But the lettuce will be followed by green beans once the weather gets hot.)
There are two main benefits to rotating your vegetables throughout your garden.
1. Rotating Vegetables Balances Soil Fertility. Different vegetables have different nutrient needs. Tomatoes, for example, require a lot of nitrogen and phosphorus. So if you plant tomatoes or other members of the nightshade family (peppers, eggplant) in the same spot year after year, the soil will not be properly balanced and will be nitrogen deficient.
By following tomatoes with lettuce and then green beans, it helps to balance the soil needs. The beans in particular will actually add nitrogen back into the soil (although they also require a lot of phosphorus).
Rotating vegetables and adding lots of manure are the best ways I know to make sure my soil stays healthy. And healthy soil grows vegetables that are very nutrient dense.
2. Rotating Vegetables Prevents Disease and Pests. Pests and diseases tend to go after plants in the same family. So by spreading out your tomatoes and peppers, it doesn't give pests a huge feeding ground of their favorite foods. It also helps to keep down the spread of disease.
It doesn't mean you won't ever see flea beetles or catepillars, but it can help to make them not a major problem.
To be honest, we don't give a lot of thought ahead of time to where and how we're going to plant things. We just naturally remember where we planted vegetables last year and try to move them to a different spot.