Making of Varga by Paul Tosca

Next open the bake dialog found in the Rendering menu set under Lighting/Shading->Batch bake (mental ray) ... go to the options and make sure you check bake shadows and orthogonal reflection, also set other options to pretty much what you see in the image below ... set the file type and size and you are ready to go

Hit the convert button and if all is set properly you will end up with a texture like below (if you can not find the textures you could always check the log or if you did not enter a path in the dialog they usually are saved in the current project folder in renderData/mentalray/lightMaps)

It won't be as crisp as an ambient map computed directly from the high version but will compute alot faster.

With the same process you can bake some quick specular passes to use for the textures:

save the same scene with another name( spec_bake or something ) and disable the FG in mental ray tab;delete the ground plane
change the material assigned to the low poly to blinn , change its color to black , specular color to white and reflectivity to 0

make a couple of directional lights and position them to light from above around the object

use the same batch bake mental ray options but it will compute super fast now ( its tracing only the specular pass) ( this time double the resolution when baking and you will donwsample it in photoshop to use on top of textures ... you will achieve better results this way )

you should end up with a texture like below and you can overlay it on your diffuse textures with screen/color dodge blending modes ( to give more variations ) ... it works great for plastic, metal, leather or other materials that have high specularity ( it wont look so good for cloth for example )

Enough said about baking ... bare with me for the last paragraph ( i kept the cool tips for the end :) )