The key to originality is personal experience. Not all of my designs are completely original, of course, but if you step back and look at the collection, you'll notice right away it's kinda different from other collections. I like to get my ideas from daily life: a hike, an interesting bug or animal, or things that just pop into my head while I'm doing the morning dishes. Everything that one sees in the world will appear differently from other people's perspectives; that's because they relate what they see to personal experience, or lack thereof.
Not all of my ideas are my own, either, as I do take suggestions from time to time, as long as they appeal to my artistic sensibilities.
Literature, too, is a great source of inspiration, especially writings about mythology, ancient cultures and traditions, and lore. Often the first thing I do after getting an idea, is to research it on the web, see what else is out there that might be similar, and what to avoid as cliche. I've heard it said that no idea is completely original. That may not be so, but true originality does seem very hard to come by.
Perhaps one of the most productive sources of inspiration for me is the process of making art. One idea, during the process of making it real, can blossom into several or many more.
I like doing serials or sets, such as the four seasons in my Solstice and Equinox collection. If the design is one where I am working with a model, he or she becomes a collaborator on the project and will add to the idea as it progresses. I might have a certain pose visualized in my head, but the model's interpretation of it will always be different.