One important consideration for planning your garden is rotating crops to manage soil fertility and control insects and disease. For example tomato, potato, eggplant varieties should not be planted in the same place from one year to the next because they are all in the nightshade plant family. Plants in the same family are all susceptible to the same kinds of insects and diseases. Remnants of these insects and diseases are left over in the vicinity of last year's crop. If you move the crops around it keeps the insects and disease from building up year after year.
Rotation is also important for maintaining healthy soil. You want to follow heavy to medium feeders that draw a lot of nutrients from the soil (tomatoes, corn, cabbage, peppers) with either light feeders (carrots, beets, onions) or heavy givers (beans, peas) that actually will fix nitrogen in the soil and enrich it. I like the methodology explained here that recommends a rotation of heavy feeders followed with heavy givers which are then followed by light feeders. I also like the chart above from Better Hens and Gardens that explains the sometimes confusing logic of crop rotatio in an easy-to-understand infographic.
Another key consideration in plotting out your garden plan is to consider what plants make good companions and like to be together and which plants are arch enemies and will fight each other all summer resulting in reduced yields.
You can go here for a run down on plants that are beneficial and antagonistic. I get kind of overwhelmed by all the information and different criteria for planting, so I have developed Craig's anecdotal, simplified companion planting plan.
Basil & Tomatoes: My first recommendation is interplant your tomatoes with basil, lots of basil. We discovered this two year's ago. We always had trouble with our basil going to seed because of the intense heat of summer, but when we interplanted them with the tomatoes the large tomato plants shaded the basil just enough to keep them in check. We had wonderful basil all summer and the tomatoes seem to really like it too.
Nasturtiums and Marigolds all around: These are my go to plants to intersperse in the veggie garden. Marigolds are legendary among organic gardeners for helping with aphids although there is some debate on this topic. One farmer friend told me about the summer he found the aphids actually feasting on the marigolds. Some people say ants will one day rule the world, I say aphids will offer stiff competition.
Plant a rainforest of peppers: Peppers are one of my most challenging crops to grow. They love the heat of summer but they also seem to like humidity which we don't have in Spokane. Planting them close together is the best trick I've learned to keep them happy.
Enlist volunteers: I have companion plants that I put in the garden every year and then I have volunteers that come back every year from previous crops. I thin them out or move them around, depending on the layout. These are known as self-seeding plants, meaning they do all the hard work of planting on their own. Some of my self-seeding companions are borage, calendula, dill, and purple coneflower. Some plants like mint are too good at self-seeding and could be considered invasive. Raspberry plants are also notorious for spreading via underground shoots.
One last consideration for your garden plan, make sure to plant you're tall shade producing plants on the north side of your plot so they don't block the sun from your shorter plants. Lettuce on the other hand is good to plant in the semi shade of sunflowers to keep it from going to seed too quickly.