Well a few ideas come to mind:
Focused electromagnetic radiation
Something along the lines of the microwave power station mentioned in other answers. Have a large maser (microwave laser) on your generation point and aim it at a secluded area of Earth for collection via a large collecting array. The collection site will have to be large due to beam scattering issues, which is actually a good thing, by spreading the power out it makes for less issues with atmospheric absorption or death ray style damages. This is currently thought to be the best and most feasible method, and has been studied fairly well with lots of information available about the particulars of how it would work.
Portable power storage
For this type of energy transfer you want things with very high energy density, which you then transport to Earth, this is not going to be very effective in general due to the high transport costs involved, especially if you need a soft controlled landing to transport it to Earth's surface, but with very high energy dense materials it might make sense.
The top of the potentials here, based on energy density, would be anti-matter, you would need a much more energy efficient method of producing it than is currently possible using particle colliders as well as other technologies for storage, transport and energy generation that do not currently exist, but if you had those a shipment of ~3 tons of antimatter could power Human society for a year (5.6x10^20 J / 9x10^16 J/kg = 6200 kg divide by two for the normal matter in the reaction = 3100kg).
More realistic options for energy carriers would be artificial hydrocarbons produced from asteroid and comet materials, or refined metals such as aluminum which can be unrefined to produced electricity. These would have fairly low energy density meaning that you would need some very efficient methods of shipping it to Earth to make it at all viable.
A possibly useful method, depending on it's availability in asteroid or other space mining, would be nuclear fission or fusion fuels. He3 has been theorized to be present in space locations produced by the solar wind, if we developed fusion power that could use this or other exotic isotopes available more readily in space I could definitely see mining for nuclear fuel as viable.
In terms of currently usable energy storage you really can't beat Uranium. If you do all of the dangerous refining and fuel processing in space transporting refined Uranium fuel to Earth would offer a very high energy transport capability.
Far down the list of theoretical methods is one of my favorites, the space elevator. The whole problem seems really easy when you can just run a long distance high voltage power line from Space to Earth!