In planning your veggie garden layout, avoid planting members of the same plant family in the same spot that they were in last year, or even the year before. This is called “crop rotation.” Members of the same family are susceptible to the same diseases and insect infestations, and utilize the same nutrients.  Planting crops from the same family in the same bed, year after year, can deplete soil nutrients, even with proper fertilizing.
For example, avoid planting members of the tomato family (tomatoes, potatoes, peppers, and eggplant) in the same place year after year.  Likewise, the cucumber family contains this and melons and squash; the onion family has, in addition, leeks and garlic; the cabbage family has this crop and many others such as broccoli, cauliflower, kale, radishes and turnips.
There are various other crop rotations you may read or hear about, but a simple one revolves around nutrient use.  Leafy crops (lettuce, spinach, cabbage for instance) need lots of nitrogen, so start them out in beds that are new or enriched well with compost and manure.  The next year, in this same bed, planting fruiting crops such as tomatoes, squash, cucumbers, and peppers.  They don’t want much nitrogen, but lots of phosphorus.
The third year, in that same bed, plant root crops such as onions and carrots.  These mainly need potassium, and grow fine if there is less nitrogen and phosphorus from the previous years.  Finally, in year four, plant legume crops such as beans and peas that actually put nitrogen back into the soil.  Add lots of compost and manure the fall of that fourth year, with other nutrients, and you’ll be ready to restart your rotation the following spring.
I garden in several raised beds, and simply rotate crops among them from year to year — perhaps the simplest form of crop rotation and the minimum you should work toward.  With this, I can usually manage a couple years between having the same crop in the same bed.  Keep a simple layout map of your beds and plantings from year to year to help in your planning. 
Dr. Leonard Perry is a Horticulture Professor Emeritus at the University of Vermont.