ffeine can act in numerous ways in the body, however, the only practical mode of action, within normal physiological concentrations, is through inhibiting adenosine receptors. These receptors are found within the brain, heart, skeletal muscle, and adipocytes (fat cells), so caffeine can influence the body in a wide variety of ways. Not going to go in a lot of detail of this, but for example, the A1 isoform of the adenosine receptor has biological actions of decreasing heart rate, so inhibiting it would serve to increase cardiac output. Also caffeine can stimulate certain brain areas like the adrenal medulla to cause adrenaline (epinephrine) to be released. The release of epi. mobilizes free fatty acids which may result in muscle glycogen sparing. However, findings on whether caffeine enhances FFA metabolism is not convincing. One study found that caffeine failed to alter the rate of FFA or glycerol (a product of triglyceride breakdown) appearance into the blood. In another, caffeine enhanced FFA levels, but failed to enhance FFA uptake into the working muscle. Currently, the theory is that caffeine enhances AI receptors of adipocytes causing lipolysis which then elevate FFA levels in the blood, followed by uptake of the FFA by the liver for oxidation, or to form ketone bodies to be cleared by skeletal muscle. Does this cause glycogen sparing? Initially, studies form that caffeine supplementation resulted in less glycogen use during prolonged exercise. Although, current research shows that this effect is minimal after the intial 3-15 minutes of exercise. Furthermore, research from the last two decades fails to show that caffeine can decrease RER, shifting the energy balance to fat oxidation over carbohydrate oxidation, prolonging time to exhaustion. So how does caffeine work to improve performance? Well studies have shown that many aspects of fatigue during endurance exercise involve a loss of electrolyte homeostasis - particularly a loss of potassium from the sarcoplasmic reticulum. This will result is less motor unit activation and force production. One study showed that by increasing epinephrine, caffeine supplementation worked to increase skeletal muscle ATPase activity and therefore more potassium in the cell, and greater muscle contractibility. Finally, the main way it has been suggested that caffeine supplementation affects performance is via A1 inhibition in the CNS which "stimulates the brain" (reducing the rating of perceived exertion, perceived pain, and increasing levels of vigor). This is of course an oversimplification, however, for the purposes of the review, which is to simply introduce the topic at hand and give a suggested protocol for optimal performance, it is not necessary to go further into.
So there are many theories as to how caffeine works in the body, but really, when it comes down to it, who cares how it is working biologically, what about performance. Jentkins et al. showed that caffeine increased work output during a 15 cycling TT by 4%. Cox et al. showed that caffeine supplementation provided a 3% improvement in a 2 h cycling trial followed by a 30 min TT. Ivy et al. showing across a variety of doses caffeine improved total work by 7% during a 2 hr cycling trial. Running? Wiles et al. showing a significant improvement in mean time during a 1500 m race on a treadmill with caffeine supplementation. Overall a meta-analysis by Ganio et al. reviewed 21 studies and showed on average caffeine ingestion improved performance by 3.2%. Now, that may not seem like a whole lot, however, in elite sport, 3.2% can be the difference between 1st and 10th. For example, if Wilson Kipsang the current marathon world record holder improved his 2:03:23 by 3.2% he would run under a 2 hour marathon.
So what's the proper dosage and timing? Most studies show that caffeine is rapidly absorbed and reaches peak plasma concentrations after approximately 1 hour. This is why most studies have protocols whereby subjects ingest a dose o caffeine rest an hour, and then exercise. While caffeine's effects "kick in" quickly, it has a long half-life and maintains high plasma concentration for 3 to 4 hours post-ingestion. Most studies use a dose of caffeine indexed for body mass, though some studies still for some reason use an absolute dose of caffeine which results in smaller body weight individuals like women to have much greater doses than men relative to their mass. One study, showed that anywhere from 3 to 9 mg/kg of caffeine had an ergogenic effect on performance. A subsequent study showed that when ingested with carbohydrate 2.1 mg/kg of caffeine was enough to improve performance, however, it appears that a dose of 3-6 mg/kg is optimal. To put this into perspective a large coffee at Tim Hortons is 140 mg of caffeine, and a grande at Starbucks is 330 mg of caffeine. So if your say 80 kg (about 175 lbs) than you need a little less than a grande at Starbucks (a tall is perfect), and a it will take a whole lot of Tim Hortons coffee. In terms of the dosing protocol, no study has compared a single pre-exercise dose to repeated doses. However, I believe that since the half-life of caffeine is 4-6 hours, and since exercise does not impair or influence caffeine's absorption, than a repeated dose in an event longer than 6 hours may be beneficial to prolong the effects. Otherwise, in events less than 6 hours one dose before the event is adequate. No studies have looked into caffeine's effects on ultra endurance events.
So what's the best strategy? For optimal endurance performance aim to ingest 3-6 mg/kg approximately one hour before the event, or, if you experience gastrointestinal discomfort with large doses of caffeine than take smaller repeated doses throughout the exercise bout. Also, studies have shown habitual caffeine users (coffee drinkers) are less responsive to the effects of caffeine - there is less of of increase in adrenaline following ingestion. The reason being that chronic caffeine consumption results in an up-regulation of the adenosine receptors within the CNS meaning a greater amount of caffeine is needed to elicit the same effects. Therefore, using a time to exhaustion protocol, one study showed that time to exhaustion was improved when subjects abstained from caffeine for 4 days compared to zero. In animal studies, it has been shown that adenosine receptor affinity was maximized after 7 days abstaining from caffeine consumption. Therefore, athletes should abstain from caffeine consumption no fewer than 7 days prior to the competition, and then have the 3-6 mg/kg of caffeine an hour before the event. This should allow enough time for withdrawal symptoms (like headaches, lethargy, flu-like symptoms) to subside. If the withdrawal symptoms are too much to handle, habitual caffeine users may just consume a larger dose of caffeine than usual before the event to experience the same ergogenic effect.
Last but not least, some people think that caffeine is a diuretic, but this is a myth. A diuretic is something that causes the body to lose more water than usual. Basically, some believe that caffeinated drinks can potential dehydrate individuals, so much that British Olympic teams have discouraged their athletes from consuming coffee, tea, etc., at competitions. Tell that to Mo Farah, who won double-gold in the 5000m and 10000m, and in an interview about what he did immediately before the race said he drank two espressos and a coffee twenty minutes before the race. In one commonly cited study advocating that caffeine has a diuretic effect, the subjects only increased urine output after they, who were regular coffee users, avoided coffee for 4 days and then drank 642 mg of a caffeinated drink - that is 5 large tim hortons coffees. Overall studies have shown that caffeine only has a diuretic effect in very high doses. Also taking a single dose of caffeine before the event like suggested in the prescribed protocol has no diuretic effect, and habitual coffee users become accustomed to its effects and therefore the diuretic effect is diminished. 
In conclusion, caffeine is a powerful and easily available ergogenic aid that consistently improve endurance performance, and has no negative effect on fluid balance. To put a little more emphasis on the 3.2% improvement documented with caffeine, just think if you are a 4 hour marathon runner having a coffee before your next marathon can shave more than 8 minutes off your time. That could easily be the difference in qualifying for boston or not, winning your age category, etc. Let's face it, if you go to any athlete and I say I can help you improve your performance without doing anything illegal and without increasing the amount of training, would you take it. Plus, it's not like I'm saying start drinking awful tasting spinach shakes, or buying expensive supplements, just brew a cup of great tasting coffee, and sip on it an hour before your race, feeling the energy fill your body and the soothing feeling of the warm mug in your hands.References
Maughan RJ, Griffin J. Caffeine ingestion and fluid balance: a review. J Hum Nutri Dietit; 16:411-420.
Burke LM. Caffeine and sports performance. Appl Physiol;33:1319-1334.
Graham TE. Caffeine and Exercise: Metabolism, Endurance, and Performance. Sports Med;31:785-807.
Ganio MS, Klau JF, Casa DJ, Armstrong LE, Maresh CM. Effect of caffeine on sport-specific endurance performance: a systematic review. J Str Condi Res;23:315-324.
