Some things to consider:
Orientation of mold block to part for machining cavities
parting line location
draft angles for part removal
gating locations to fill thin areas and avoid trapped air pockets
cooling rates for different cross sections
When designing a mold in Inventor, you should use derived parts.
I use Inventor 10, and I can't remember if derived parts are created the same in IV 8.
Basically, you would model your cavity blocks, without the part cavity. Then create an assembly of both your top and bottom cavity blocks with your part.
From there, start a new ipt file and close the sketch. You'll have an option for derived parts in your part features menu. After selecting the Derived Parts command, you need to select the assembly you just created from the Open file dialog box. You then have a dialog box that will allow you to subtract the part from your cavity block. Remember, this is an ipt file, so you can model only one cavity block in this file. You'll need to repeat this process for the other cavity block in a separate ipt file.
This is just a quick and dirty overview. For more info, refer to the following website (www.sdotson.com). This includes a lot of useful inventor info. Go to the Tutorials page and you'll find two tutorials on Derived parts.