MANY PLANTS DIE HORRIBLY if their feet are wet. If your site is mostly wet, as mine is, you can address this easily by planting the species--typically those that grow on or near wetlands--that will tolerate wet soil. You can get a lovely garden this way; in fact I find that I'm designing more and more of my garden in such a way as to accommodate wetland plants.
But if there are "dry feet" plants you can't live without, the only solutions are either raised beds or a rockery (which is really a type of raised bed). By elevating your planting surface above the soil's natural level, you ensure that water will drain away from plant roots. If you're a cold-climate gardener, an additional benefit is that the soil in a raised bed generally warms up faster than does the natural soil.
Many plants love this type of environment. Most vegetables and culinary herbs require it, as do many beautiful plants that are native to North America, such as purple coneflower. Many of the best butterfly plants are dry-footers; if you want a butterfly garden, I definitely suggest finding or making a sunny, well-drained spot for it.
A conventional way to make a raised bed is to build one out of lumber, but lumber costs money, might be treated with toxic chemicals, and may not have been harvested sustainably.
Fortunately, there are easier ways to make raised beds. For instance, for my butterfly garden here in the Northeast, I was able to take advantage of regrading work that had to be done anyway. When I got here, the basement flooded regularly. A local backhoe expert fixed that for me by changing the drainage pattern in my front yard, and in the process he piled dirt in front of my house. The photo to the left shows the work being done in August 2003. I mixed manure into the soil and planted in 2004, and the photo above shows my butterfly/hummingbird garden in bloom in June 2006. Plants such as salvias, penstemons, and dianthus are thriving in the sun and good drainage. (Reflected sun from the white house helps to make this spot even warmer than it might otherwise be, while the house itself helps to shelter the site from wind. This is a perfect spot for a butterfly garden.) An added advantage of this bed is that, because it's sloped, the plants are more visible than they otherwise might be.
But I don't want to give the impression that you need a backhoe to make a raised bed! The point of this example is that you can take advantage of almost anything that happens on your property, even mini-disasters like flooding in the basement, to make good planting sites. Lemons into lemonade and all that.
The photo below shows a raised bed I made for my garden in the Pacific Northwest. This property had a long gravel driveway that I didn't particularly care for. I also had a lot of yard waste that could have been turned over to the county for composting--but why give away all those good nutrients? So I piled the yard waste on the driveway, let it sit until it had compacted a bit, added some good-quality dirt, and planted the mound with dry-feet plants such as artemesias and sedums. The logs that define the edges of this mound were an afterthought, inspired by a raised bed my friend Brett Johnson made out of some unwanted firewood. An arborist I knew offered me the pieces of an alder he'd cut down, and I grabbed 'em.
Some sort of edging definitely makes a raised bed look tidier. I use whatever I have handy--logs, rocks, even bits of broken concrete. But edging isn't absolutely necessary. For utilitarian purposes, such as a vegetable garden, a simple mound of soil will work remarkably well. The photo below shows one of my vegetable beds in the process of being made by mounding up soil I purchased by the truckload from a local composting facility. Thanks to these beds, I am generally able to start planting and harvesting earlier than my neighbors who have not adopted the raised-bed strategy. By using soil that had been mixed by professionals specifically for vegetable gardening, I was also able to get a good harvest my first year, without having to spend several years building soil.