I think ion engines could be a possibility.
Small spacecraft landing on the surface powered by nuclear batteries. Then use the thrusters to slowly change it's path. It would take time though,
and in a worst case scenario, time would not be on our side. In that scenario, we would need brute force.
www.spaceweather.com... Tracks a lot of asteroids, and even provide an applet to track them with, and data to find them in a telescope - quite
nifty actually, and a great source for space related info.
I really don't think launching a lot of mass and putting it in orbit is feasible at the moment. The sad thing is that we need a pretty devastating
strike before anyone starts looking into this