Joseph Campbell claimed that there are different 'plains' of consciousness, and he used these emotions to explain such things. Walking through an urban city that has all the sound and commotion that are want to happen when thousands of people congregate, bustling about to wherever they need to be. But if you walk up to that cathedral and walk through the doors, suddenly you leave that plain of consciousness. The hustling and fast-paced tumult is suddenly replaced with a peace and calm mind. Often, you can even hear the sound of hymns being sung in the back of your head. It's hard to imagine these two completely opposite states of mind can be so readily associated with one another.
Yet this is a technique that is used in every day life. Taking myself as an example, I as a writer try my best to inspire my own thoughts using music and ambient noise websites. For the last several sessions I've been sitting down to write Dawn of Night, I've been listening to birds and wind, along with some violin pieces that have a 'forest' theme. Also, I always look for a picture to associate with the blog that reflects the information as well as I can, and looking at forest pictures helps, too. Basically, I do whatever it is I can to get myself into the mindset of what I need to do.
In general, when I'm on the computer I listen to things that reflect my mood, or reflect the mood I want to be in. Some time ago I wrote about mood congruence theory, which basically states that we find everything in correspondence with our current mood appealing. When you're sad you want to listen to sad songs and vice versa. But it also means that listening to things will help put you in that mood. That is why I believe listening to evocative music and sounds helps so much with my writing.
If I were to write while listening to my favorite bands (most of which happen to have some depressing songs and lyrics), then my writing would reflect that even if I didn't intend for it to be that way. So the next time you're trying to work on something or change your mood, ask yourself this: Is there anything you can do to physically change your mood? It's a simple question, but don't underestimate how much difference the little things can make.