Mine negawatts: The cheapest source of new energy is not using it - increase efficiency and you can slash the growth in energy demand. See part two of the interview with Michael Brylawski of RMI for his related comments on how people don't want extra kilowatts, they want hot showers and cold beers.
A good example of where to find negawatts is right above you - replacing standard incandescent light bulbs with compact fluorescent light bulbs as is being proposed here in Australia. Similarly, make a car lighter and you can make it more efficient, or faster, or both.
Electrify: It's easier to find clean, renewable sources of electricity than it is to find clean, renewable liquid fuels which leads to an obvious conclusion - shift almost all energy use to electricity. That's not to say that there isn't a lot of room for renewable fuels like ethanol and biodiesel in the mean time, but long term, the electricity grid is just more efficient.
Kill coal: "Coal is the enemy of the human race." Coal fired power stations are responsible for a huge percentage of the world's total greenhouse gas emissions. Coal also causes environmental damage where it is mined and air pollution where is burning. Cleaning up coal is likely to make it financially non-viable in comparison to green technologies such as wind.
Upgrade the grid: Focus on energy storage and decentralisation through renewable sources like wind, solar, geothermal, hydrokinetic, cogeneration and biomethane. Developing good power storage technology is important to even out the supply intermittency of renewable energy sources. Examples of storage Roberts mentions include battery technology (both lithium and nano), hydrogen fuel cells, pumped storage, molten-salt storage and ultracapacitors.
Analysis: As Michael Brylawski said, "if the ultimate efficiency is, say, 10 percent of turning coal into refrigeration for your beer, every unit of energy saved at the fridge saves ten times that in coal." I especially like the concept of mining negawatts - you can start today by using compact fluorescent light bulbs.