Method 1: Keeping the normals
According to the Max help, this can be done by breaking up your mesh, selecting all the parts and then applying an Edit Normals modifier and selecting the normals across the seam, and then just hitting Unify. This has never worked for me. If someone knows why, I'd love to hear. I can only presume it's a bug in Max 6... If this worked, these other workarounds wouldn't really be necessary.
The next way to do this is the easiest by far, and also the quickest for setting up. In order to do it, you will need a script written by Maurizio B.G. The script is called Normal Thief 1.0, just do a search for Normal Thief and you should find it at that link.
Now to use it...
Install it, and launch it with Max script in the Utilities tab. You should get the following dialog:
Click on image to view larger
Now we need to duplicate our original mesh, and cut it up by selecting faces and detaching them as we see fit. The important thing here is that these meshes do not move away from their original location in the scene, (they should still be located directly on top of the original mesh)
Once done, we click on , select the original mesh (press H on your keyboard to get the Select dialog up), then we select our new meshes (all the cut up meshes), and click Action on the Normal Thief dialog.