The only consistently reliable way found to slow the aging process across a large number of species is calorie restriction (CR). Cut calorie content of a diet to 30% less than an organism naturally eats and max and average lifespans will typically go up by 20% or 30%. CR is practiced by relatively few people because it is difficult to do and has some side effects (eg a gaunt appearance). A human study of the long term effects of CR has not been done because it would take a very long time to prove that CR really does increase lifespan (though shorter term studies have been done). The life extending effects have only been demonstrated in shorter lived species because its much easier (ie it takes a lot less time) to do lifespan experiments on shorter lived species. However, the physiological effects that happen during CR in other species (eg lower blood insulin and other factors easily measured in blood tests) also happen to humans on CR and human metabolism has enough in common with other species that it is reasonable to expect that CR will increase human life span.

As Scientific American reports in a lengthy interesting article, species more like humans are being studied on CR regimes and preliminary results look promising:

The rat findings have been replicated many times and extended to creatures ranging from yeast to fruit flies, worms, fish, spiders, mice and hamsters. Until fairly recently, the studies were limited to short-lived creatures genetically distant from humans. But long-term projects under way in two species more closely related to humans--rhesus and squirrel monkeys--suggest that primates respond to caloric restriction almost identically to rodents, which makes us more optimistic than ever that CR mimetics could help people.

The monkey projects--initiated by our group at the National Institute on Aging in the late 1980s and by a separate team at the University of Wisconsin- Madison in the early 1990s--demonstrate that, compared with control animals that eat normally, caloric-restricted monkeys have lower body temperatures and levels of the pancreatic hormone insulin, and they retain more youthful levels of certain hormones (such as DHEAS, or dehydroepiandrosterone sulfate) that tend to fall with age.

Some scientists believe that it may be possible to use a pharmacological agent to flip metabolism into the mode that CR puts the into. As this article indicates, the search is on for compounds that will emulate the metabolic changes that CR causes. The most thoroughly investigated compound to date is 2-deoxy-D-glucose. The problem with it is that its therapeutically effective dose in rats is very close to its toxic dose. If a safe and effective compound can be found then it will be possible to achieve most of the benefit fhat CR provides in terms of slowing aging without the need to feel hungry or have a gaunt appearance.

Dog lover please note that, as recently demonstrated with Labrador retrievers, CR is beneficial for dogs: (same article here)

The researchers found that the median life span--the age by which half of the dogs had died--was nearly 2 years longer among the calorie-restricted dogs (13 years, versus 11.2 years). The dieting dogs also tended to go longer without needing treatment for chronic conditions--age 12, on average, compared with age 10. In both groups of animals, osteoarthritis was the most common medical problem, but the calorie-restricted dogs developed the condition an average of 3 years later than their litter-mates.