The below information was taken directly from the Muscle and Fitness article Ah! There's the Rub by Len Davis and Jim Stoppani.
Aminophylline decreases the activity of enzymes that inhibit fat-burning within fat cells. Studies show that topical creams containing aminophylline can reduce fat levels directly beneath the skin. When combined with topical ingredients that increase fat-burning through different mechanisms, aminophylline has been shown to be even more effective at decreasing fat stores.
Forskolin comes from the herb coleus forskohlii, a traditional Indian herb used in Ayurvedic medicine. It also plays an important role in fat-burning within fat cells. Oral forskolin supplements have been shown to reduce bodyfat while boosting testosterone levels. Topical forskolin creams have also helped study subjects lose subcutaneous fat when used consistently for four weeks.
Caffeine is probably the most clinically studied and most commonly used oral supplement for fatburning. A study as far back as 1979 found that caffeine is also well absorbed through the skin, as evidenced by increased urine levels of caffeine after topical administration in humans. Because of caffeine's high permeation through the skin, it's used medically in certain conditions for increasing delivery of caffeine in the body. Topical caffeine applications also increase blood flow -- important for burning fat since limited blood flow can be a limiting factor in fat usage.