Well, one way to do it is to turn off your subdivion modifier, or reduce the number division in it. [Assuming you've not gone ahead and applied it permanantly]

Another is to actually use a whole new version of the mesh called an "LOD" [level of detail] model that gets swapped in as the object gets further away from the camera. Its not uncommon in some cases to have three or four of these for an object, set to be used for varying distances.

The LOD's can be created a couple different ways. Either completely remodeling the mesh the way you'd retopo a high poly sculpt to a production level model, or using a polygon reducton tool. In Blender there's a modifier called "Decimate" that'll do what you're looking for, tho be warned, any automated polygon reduction tool has the potential to make an complete mess of your mesh.

 

This is kinda general, but hopefuly it gets you on the path you're looking. :)

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