This think jmed's link has the right idea.
The key to desensitization, in my experience at least, is to be able to force calmness during the whatever is causing the nervousness/stress/fear durring exposure. Start off with the smallest stimuli that invokes fear and do your best to be calm and relaxed. Deep breathing at first and latter activities that require some attention like reading, cooking, stretching, etc. After the current level of stimulus does not interrupt these activities increase the level of stimulus and repeat.
If no progress is made in a month I would recommend continuing the effort, but shifting focus to conquering smaller fears first. Take advantage of real life situations that make you nervous/stressed/fearful and do your best to calm yourself. Do scary/action movies get your heart pumping? Go to a movie let yourself get wrapped up in the movie and then try to calm yourself down.
Okay, so for several years, I've been doing a fairly good job of excising out most of my irrational beliefs and fears.
The one that remains, however, is my fear of bugs and spiders. I'll SCREAM if I see one, dead or alive. And if there's one in my food, then I'm going to throw the entire plate out.
Is it common for LWers to have irrational reactions to bugs and spiders?
I know that there is "desensitization therapy", as highlighted by a Scientific American Frontiers episode a decade ago (where gradual exposure to the stimulus - a spider) - can desensitize a person with arachnophobia over time. But this can really only happen in controlled settings. In uncontrolled settings, such interaction can give you nightmares.