Step 1: Go Get Stuff
(x1) Aluminum foil roll
(x1) 123D Catch
(x1) Digital camera
(x1) 3D modeling software (I used 3DS, but you can use something free like 123D)
(x1) Very expensive high-resolution Objet 3D printer (Or friend with access to a printer... or featured entry in the Make it Real Challenge... or a Shapeways account)
Step 2: Catch Yourself
In some hours you should receive back a 3D model.
Clean up the model as necessary (i.e. remove any weird spiky bits).
Step 3: Boolean Subtract
Import your face STL from 123D Catch and close the mesh to make it a solid object by closing the mesh.
Create a new cube and then subtract your face from it using a boolean subtract operation.
This should leave you with a cube with the inverse of your face.
This will be the inverse face mold that you will 3D print.
Step 4: Fix the STL
I used NetFabb to repair all of the errors in the STL. This program can be used for free and is quite good at repairing STLs. However, any program capable of repairing errors in STLs will do.
Step 5: Print
If you don't happen to have a fancy 3D printer, get someone else to do it for you.
Step 6: Make the Cast
When you are done, pull the two apart and you now have a mini replica of your face.