We found this article at Organic Gardening very helpful for learning the basics of companion planting. From taller plants offering shade to shorter ones (and their root systems fitting well together underground, making for more efficient use of garden space), to certain plants replacing nitrogen in the soil that others remove, companion planting is all upsides and no downsides.
We're taking it to heart: since we're planting our cucumber and squash plants already, we're looking to get some nasturtiums, which are said to repel the bugs that like to attack squash, in the same bed. Do you have any experience with companion planting in your own garden? Did you notice a difference between companion planting and more spontaneous planting?