Agree with Rocketman. In 123D Design you
1.  make the parent object you want to derive the mold from,
2. make a separate cube,
3. slide them together about halfway then
4. use the Combine Subtract tool to exclude overlapping parts. See attached photo for result.
5. Make another mold for the other half of the parent object and you're set.

Now, you need to think about how you design the parent part. Complicated parts with cutout, hollows, bridges and such may be hard to make a mold from i.e. where do you split the mold?

Just curious, but wouldn't it be easier to surround your entire original part with a cube then subtract it? You could then easily add rods to create "holes" after subtraction (or do at with the original model from the start) to have the fill / over fill holes built in.

Splitting in 123D is ridiculously simple even if deciding where you need the split isn't , you can just take a cube of any size making sure the axis you want to split on is like .0001 thick, move into place and merge / subtract. I think it would be fairly simple to get the mold apart when you can make as many "cuts" in it to release as you need. Sliding "pins" from the outside of you cube that you have copied (so you have an identical one in the same place) then doing a subtraction would make it fairly easy to have matching pins and hole to hold a mold together.

Use Meshlab for this process. It’s free and can be found here. Meshlab is a full-featured tool but I mostly use it for reducing polycounts. Start Meshlab up and load your STL by clicking on File->Import Mesh…
Next take a look at the face count. It’s circled in red in the above picture. Right now it’s at almost 1 million. I want to reduce it down to well under 100K for the first part of our process.
Click on:

Filters->Remeshing, Simplification, and Reconstruction->Quadratic Edge Collapse Decimation

Set the number of faces to something that still looks reasonable when you hit “Apply” but is below 100K (I chose 60K). Hit “Apply”.

Your Model is now decimated down to a workable number of faces. If you skip this step, some of the software used in later steps won’t load the model, or will crash when you try.
Step 2. Is it manifold?

Now we need to verify our model’s manifoldness. Quadratic Edge Decimation generally produces good-looking results but frequently breaks manifoldness. The attitude bunny I’m using was manifold before the decimation, but now is not. Here’s how you tell:

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Click Render->Show Non-Manif Edges

     then

Click Render->Show Non-Manif Vertices

Filter->Cleaning and Repairing-> Remove Duplicate Faces

   Filter->Cleaning and Repairing-> Remove Duplicate Vertex

   Filter->Cleaning and Repairing-> Remove Faces from Non Manif Edges

Once that’s done export your mesh as an STL even if it still isn’t perfectly manifold.

If after selecting all of the above your model still isn’t manifold, you’ll have to load it in Netfabb Basic (also free) and attempt automatic repair from there.

If Netfabb basic won’t fix it you may need to reduce the polycount further and repair it by hand in Sketchup. This is tedious and not for the faint of heart.

Speaking of Sketchup, it’s a great tool for taking a look inside our mesh to see how the insides look. It’s free and you can download it here.

You’ll need to install an STL importer/exporter. There’s one available in the Sketchup plugin library. Once you’ve done that, you’ll be able to import your model and have a look.

Step 3. Get rid of internal structures

This used to be a long and tedious process but now there’s an easy and free solution. Autodesk’s Meshmixer tool is free and can take care of this for us.

Now, you need to think about how you design the parent part. Complicated parts with cutout, hollows, bridges and such may be hard to make a mold from i.e. where do you split the mold?

+1 ...bookmarked your tutorial.. i learn some hints.. thanks

Does this mean we can expect some scantily-clad fantasy chocolate molds soon?

Nice write up.

Thanks!

Here's a tutorial on how to turn a 3D object into a 3D printable mold!
http://bit.ly/1H7rEgN

You'll need:
- Meshlab
- Autodesk's meshmixer tool
- OpenSCAD

Cheers