"In all the discussions of creating an Earthlike environment on some other planet, I almost never see discussion of changing a planet's orbit to put it into a similar distance from the Sun as the Earth.

The energy required is of course massive but, if there's some way to do it, the payoff would be immeasurable.

I'm fully convinced that the only way Mars can be made inhabitable is through this method. The increase of solar radiation on its surface from bringing it close to the Sun would put its temperatures above the melting point of ice freeing the huge reservoir of water we seem to now believe lies beneath its surface. From there it's a matter of finding ways to extract oxygen from that water to rebuild the atmosphere. "

I think a large reflector at Mars L-2 would be much easier- though that would not be easy- it would somewhere in the order of a trillion dollar project.

"Obviously you can't just attach rockets and tug it where you want it. The energy required is more massive than we can provide. Perhaps manipulating smaller but still massive objects in our solar system for directed impacts on the surface could change the planet's orbital velocity enough to move it closer to us. "

The energy involved in Mars orbit, would be it's mass halved times it's velocity squared.
Mars mass is 10^24 kg
And it's orbital velocity is 24,100 m/s.

Obviously one wouldn't need the amount of energy required to stop Mars' orbital velocity, but it would some fraction of that amount of energy- like somewhere around a 1/100th of that energy.
As wild guess I would say it's about the same amount of energy require to lift the Moon's mass from the earth's surface into an earth orbit.

As far as impacting Mars with space rocks to change Mars' orbit. As general rule space rocks from beyond Mars orbit will add more orbital energy, and those within Mars orbit will have less orbital energy. Or if one transferring massive amount of cargo- those coming from beyond Mars orbital distance will add orbital energy and cargo from destinations more towards the Sun [Earth, Venus, Mercury] will subtract from Mars' orbital velocity.

For instance if one were transferring large quantities of Venus CO2 to Mars [and doing it with minimal energy trajectories- which you would do with this freight type stuff {as compared to something requiring a fast transit time} the result [if one talking about a very significant amount of mass] would be to decrease Mars' orbital distance.

A different way to look at this, assuming one has some strong interest in decreasing Mars orbit, is one could use Mars as gravity sling shot to make objects go further out [or escape from the solar system]. In other words if you had a comet, and wanted to eject it from the solar system, you might use Mar's orbital energy to do this. And/or one could also probably find a orbit/trajectory for massive objects which would cause Earth or Venus to go outward and Mars to go inward in their orbits- sort of like a cycler:

"A Mars cycler (or Earth-Mars cycler) is a special kind of spacecraft trajectory that encounters Earth and Mars on a regular basis. The term Mars cycler may also refer to a vehicle on a Mars cycler trajectory. The Aldrin cycler is an example of a Mars cycler."
http://en.wikipedia.org/wiki/Mars_cycler

Now, Mars' orbit is quite eccentric. It's orbital distance from the Sun varies from 206.6 to 249.2 million kms.
I could argue that it's a good thing that Mars has an eccentric orbit- one might even want it to be more eccentric.
But if you want more solar energy hitting Mars, making Mars orbit so it's less eccentric and so therefore say, it's always about 206 million km from the Sun, this would have a significant effect- maybe adding around 10 C to it's average temperature.

Getting back to impactors. Suppose one were interested in warming Mars for a short time period of hundreds to a thousand of years [a blink in time geologically, but quite significant chunk of time for humans]. The heat generated from impactors could useful in this regards.
In other words, a hot spring type region on Mars, would be a choice location on Mars. Part of this is because, it would a good location to find alien life but another reason is it could be more habitable condition for human settlement.

So, impactors would have some global effect upon Mars, but they could also have a regional effect, and this regional effect would be more important to humans in the relatively shorter term.