Most people report a very good success ratio by cutting down caffeine intake at the rate of 1/2 cup of coffee a day. This is known as Caffeine Fading. Alternatively, you might try reducing coffee intake in discrete steps of two-five cups of coffee less per week (depending on how high is your initial intake).

If you are drinking more than 10 cups of coffee a day, you should seriously consider cutting down. The best way to proceed is to consume caffeine regularly for a week, while keeping a precise log of the times and amounts of caffeine intake (remember that chocolate, tea, soda beverages and many headache pills contain caffeine as well as coffee).

At the end of the week, proceed to reduce your coffee intake at the rate recommended above. Remember to have substitutes available for drinking: if you are not going to have a hot cup of coffee at your 10-minute break, you might consider having hot chocolate or herbal tea, but NOT decaff, since decaff has also been shown to be addictive. This should take you through the works without much problem. Some other people quit cold turkey.

Withdrawal symptoms are quite nasty this way (see section below) but they can usually be countered with lots of sleep and exercise. Many people report being able to stop drinking caffeine almost cold-turkey while on holidays on the beach. If quitting cold turkey is proving too hard even in the beach, drinking a coke might help.

What are the symptoms of caffeine withdrawal? Regular caffeine consumption reduces sensitivity to caffeine. When caffeine intake is reduced, the body becomes oversensitive to adenosine.

In response to this over sensitiveness, blood pressure drops dramatically, causing an excess of blood in the head (though not necessarily on the brain), leading to a headache. This headache, well known among coffee drinkers, usually lasts from one to five days, and can be alleviated with analgesics such as aspirin.

It is also alleviated with caffeine intake (in fact, several analgesics contain caffeine dosages). Often, people who are reducing caffeine intake report being irritable, unable to work, nervous, restless, and feeling sleepy, as well as having a headache.

In extreme cases, nausea and vomiting has also been reported.

References.

Caffeine and Health. J. E. James, Academic Press, 1991.
Progress in Clinical and Biological Research Volume 158. G. A. Spiller, Ed. Alan R. Liss Inc, 1984.

What happens when you overdose?

From Desk Reference to the Diagnostic Criteria from DSM-3-R
(American Psychiatric Association, 1987): Caffeine-Induced Organic Mental Disorder
305.90 Caffeine Intoxication

Recent consumption of caffeine, usually in excess of 250 mg.

At least five of the following signs:

restlessness
nervousness
excitement
insomnia
flushed face
dieresis
gastrointestinal disturbance
muscle twitching
rambling flow of thought and speech
tachycardia or cardiac arrhythmia
periods of inexhaustibility
psychomotor agitation

Not due to any physical or other mental disorder, such as an Anxiety Disorder. Basically, overdosing on caffeine will probably be very very unpleasant but not kill or deliver permanent damage.

However, People do die from it.

Toxic dose

The LD_50 of caffeine (that is the lethal dosage reported to kill 50% of the population) is estimated at 10 grams for oral administration.

As it is usually the case, lethal dosage varies from individual to individual according to weight. Ingestion of 150mg/kg of caffeine seems to be the LD_50 for all people. That is, people weighting 50 kilos have an LD_50 of approx. 7.5 grams, people weighting 80 kilos have an LD_50 of about 12 grams. In cups of coffee, the LD_50 varies from 50 to 200 cups of coffee or about 50 vivarins (200mg each).

One exceptional case documents survival after ingesting 24 grams. The minimum lethal dose ever reported was 3.2 grams intravenously, this does not represent the oral MLD (minimum lethal dose). In small children, ingestion of 35 mg/kg can lead to moderate toxicity.

The amount of caffeine in an average cup of coffee is 50 - 200 mg. Infants metabolize caffeine very slowly.

Symptoms

Acute caffeine poisoning gives early symptoms of anorexia, tremor, and restlessness, followed by nausea, vomiting, tachycardia, and confusion. Serious intoxication may cause delirium, seizures, super ventricular and ventricular tachyarrhythmia, hypokalemia, and hyperglycemia. Chronic high-dose caffeine intake can lead to nervousness, irritability, anxiety, tremulousness, muscle twitching, insomnia, palpitations and hyperreflexia.

For blood testing, cross-reaction with theophylline assays will detect toxic amounts. (Method IA) Blood concentration of 1-10 mg/L is normal in coffee drinkers, while 80 mg/L has been associated with death.