Here:

http://archives.cnn.com/2001/TECH/space/02/05/earth.move/

To quote:

The researchers' theory is a twist on the "gravity-assist" technique used to send spacecraft to the outer planets. The team says that by shooting a large object (such as an asteroid about 62 miles across) past the Earth, the planet could be gradually pulled away from the Sun. It would take thousands of encounters to make a difference. One million encounters would move the Earth out 41 million miles, or about 50 percent farther from the sun than it is today.

But the researchers say that if the technique is repeated an average of every 6,000 years, the orbit could be increased to keep pace with the Sun's increasing brightness. The result, they say, would be to keep the Earth habitable for up to an extra 5 billion years.