Though most commonly seen around Christmastime, wreaths are a great form of decoration for all seasons. With a little creativity, you can create your own wreaths for each season.
Winter wreaths are already the standard. They incorporate elements and colors that most people associate with winter and Christmas: pine branches, red berries, etc. Pine cones, candy canes, and red ribbon can make simple winter wreaths a little more interesting.
Like for a winter wreath, for a spring wreath you want to incorporate things that remind people of spring. Instead of using pine branches for the main body of the wreath, consider using something more spring-like: branches from leafy trees, especially blooming ones (flowers are an essential part of spring). Try cherry blossoms or something similar if you can obtain it. Ribbon is always a nice addition as well; go for a 'springy' color such as yellow, pink, or light blue.
For a summer wreath, you should also use deciduous, leafy branches. Instead of flowers, add something like fake butterflies, or anything else that reminds you of summer (any accessories can be attached with wire or a hot glue gun). If it's around the Fourth of July, small U.S. flags or red white and blue ribbon are a great idea.
An autumn wreath looks most seaonal when it is made out of fallen, colorful leaves. Don't worry if you can't find enough good leaves or they crumble when you try to put them together -- many stores carry fake ones during the fall. There's more to an autumn wreath than just leaves of course. Pine cones, late-blooming flowers, dried fruits and berries are all common additions to fall wreaths. In moderation, adding straw can also contribute to the "thanksgiving" feel that many people associate with autumn.