My garden planning has morphed a bit over time. For the most part, I follow the squarefoot gardening guidelines and spacings. As a rule of thumb, that's a really good starting point for small plot planning. The trick for us year round gardeners though is with succession planning. Having a bunch of space going bare can be a huge disappointment.
With limited space it's a good idea to always have something waiting in the wings. And with SQFG another thing to consider is "tweeners." These are things that you can "shoehorn" in to fill up any gaps or shady places. Lettuce, garlic, bunching onions and small carrots such as "short 'n sweet" make for great tweeners.
A good plan for small plot gardens also includes crops that can do double or triple duty. For example, beet tops can serve as salad greens as they mature, and eventually will be harvested for their roots. Nasturtiums are attractive flowers that are also very good in salads, as are the leaves.
Plant what you like, plant something new, experiment, and have fun.