The big secret to urban gardening is the use of square foot gardening.  This means that your garden is set up in a square grid and you plan your garden within that grid.  Based on the seed size, you can put a specific number of seeds in that grid.  You can also co mingle different plants - based on whether or not those plants "get along".  

The benefits of the square foot gardening method is that you can plant a lot of produce in a small space.  This allows you to utilize all the space in your garden and take your garden from spring to fall.