Post feedback on how you did, with pics/screenshots so others can learn too

The way I do it is using slic3r 1.1.4.

My procedure to cut large models, making sure errors are minimized is:

1 - Open Slic3r, click File - Repair STL File. Select your STL, and it will automatically generate a "fixed" OBJ file. (Sometimes it won't be able to fix everything, and you might have to manually fix things, but it's rare)

2 - In Slic3r, click Add, or drag and drop your OBJ onto your plater. Then click Export STL. This will be your full fixed model.

3 - On the plater, double click your object, or select your object, and click the View button.

4 - It might take a while for the 3D view to come up. Be patient. When it's up, you can slide the Z slider to indicate where to cut.

5 - You will now have a model sliced in two at the point you specified. Click Export STL to get your cut model.

6 - For BIG models, I will slice each section I want, delete the other models, keeping only the section I want to save. That way I have each section in a different STL file.

Bonus tip: If you have less than 12GB of memory and are trying to slice a very high detail large model, you may notice your slicer will crash once it runs out of memory.

Coupla things that might help:

- Print settings - Advanced - Other -> Set resolution to 0.1 or 0.2
- Print settings - Advanced - Other -> Set Threads to 1. It will take a lot longer to slice, yes. But having 4 threads will take up roughly 4 times as much memory, sometimes more, sometimes less
- Reboot your computer, kill all non essential startup services/apps

If you're looking for some fairly large, open source STL's to test out, try thingiverse:
http://www.thingiverse.com/thing:54994