Desensitization is the name of the game. Some golden rules of desensitization are: Accurately identify the source of the fear.
Prevent exposure to the fully blown fear stimulus during retraining.
Be able to control the fear-inducing stimulus so that it can be presented at low, incrementally increasing levels of exposure (e.g. for fear of strangers - a volunteer fear-inducing stranger who will agree to present himself/herself at varying distances).
Test the fear-inducing stimulus to make sure that it does, in fact, produce the fearful response. Then wait a few days before commencing the program.
Present the offending stimulus at a low level of intensity.
Gradually increase the challenge by decreasing the distance between the cat and the feared stimulus, by increasing the volume of a sound recording, or by adding new dimensions to the fearful situation.
Do not advance consecutively through such a program of desensitization; instead proceed in a random fashion. As long as the cat remains calm, for instance, expose the cat to a stranger at 20 feet, then 12 feet then 20 feet, then 8 feet, and so on.
Though the distance may vary in either direction between sessions, over time there should always be a progression (i.e. in the example above, the stranger is being accepted at progressively closer distances).
If a problem occurs at any stage of the program, return to an earlier stage of the retraining process, always finishing a training session on a positive note. The following day the session can be reinitiated at a low level of exposure, which is subsequently increased to, and finally through, the former upper limit of acceptance.
Training should preferably be conducted every day, however, training sessions 2 to 3 times weekly sometimes suffice.
Desensitization is usually performed in conjunction with counterconditioning (with cats, this almost always involves using delicious food to change the cat's perception and behavior at each stage of the reintroduction process).
The most difficult cats to treat are those with "global" fear, meaning simultaneous fear of multiple cues; animate, inanimate, and situational. Cats of this disposition are almost impossible to desensitize to the multiple stimuli that trigger their fear. They are the "Nervous Nellies" of the feline world and are probably best treated medically to alleviate the impact of negative experiences that pervade their world. Even these cats, that seemingly have nothing to fear except fear itself, can be brought around by means of judicious anti-anxiety medication and subsequent weaning of the medication over time. The latter process should be conducted only under the strict guidance of a veterinarian, perhaps with input from a veterinary behavioral specialist.
Many anti-anxiety and anti-depressant drugs have been employed to facilitate retraining – with varying degrees of success. The best are (in order): Buspirone (BuSpar®)
If whatever frightens your cat can be consistently represented in an attenuated, non-threatening way, gradual reversal of the fear will result. The principle is similar to that involved in homeopathy – that of treating a condition by administering small carefully gauged quantities of things that excite the symptoms. Behavioral medicines can be helpful in ameliorating entrenched fears and fears that are "global" in proportions. Finally, there is a very good chance of rehabilitating cats with excessive fearfulness, especially if the fear can be clearly identified, is discrete, and can be easily isolated and controlled.