sorry I missed this thread (hopefully you should have your bits by now), I've been meaning to post a thread on how I do my 2 part press molds but never got round to it.
First, I lightly coat the model(s) to be molded in vaseline (cheap mold release), then press it into any old air dry clay and take a lot of care to make the clay flat and at exactly the 1/2 way point of the model. I then get some toothpicks, chop them up and coat one end in vaseline. The vaseline end gets stuck into the clay. 5 to 6 of those equally spaced around the model works pretty well. Let the clay dry
Then I make up the silicon molding stuff (I use http://www.makeyourownmolds.com/silicone-plastique
but anything similar will do) and work it into the model, usually as a thin "skim" cover first, then build it up until it's ~10mm thick. Let it dry overnight.
Very very carefully break the clay off the model (this will be where you curse yourself for forgetting the vaseline). I usually heavily score the clay and break it off in chunks. If you try and pull it off in one piece you'll pull the model out and it's a PITA getting it back in.
Turn the mold over so that the half encased model is pointing up. Clean any clay off the model and recoat it, the silicone (v. important) and the ends of the toothpicks with vaseline. Cover the model with silicone as above, let it dry overnight.
Before splitting the silicone mold, cut 3 to 4 notches around the mold where the 2 halves join. These are very useful as alignment marks to make sure the 2 halves go back together as they should. Split the mold - you may need to score it with a craft knife around the join. Go ooh and aah about how cool the mold looks.
Put it back together and use some of that air dry clay to make an external support for one side. All this is is a piece of clay that's flat on one side (I use baking paper so it doesn't stick to the table) and molded to the mold on the other side. Let it dry overnight and do the next side the following day. These are really useful so that you don't oversquish the mold and make concave models. It also give your u-grips or vices or whatever something flat to press on. Write the mold and the side on the clay halves as they're a royal pain to match up if you have a box full of them.
Clean the mold and give it a light coat of vaseline. Mix greenstuff and milliput superfine/ magic sculpt (1st is great but expensive, 2nd is just as great and 1/2 the price) in a 2:3 ratio. GS gives the model flexibility but is a complete git to use, fine clay gives the model detail but is very brittle on its own. Press it into one half until it's flush with the surface of the mold, plus a teensy bit extra. Give it a finger dab worth of water to keep it wet. Do the same to the other half. Line the 2 halves up, press them together by hand (if you see a lot of clay come out, you've used too much) and then hold the whole sandwich together with some u-clamps/ squeezy clamp things (I got a bag of 15 or so of different sizes for $10) overnight. Carefully peel one half of the mold off (one half will always come off more easily), then pop the model out of the other half by working around the whole mold.
Takes a lot of work but the results are pretty impressive and once it's done, the molds last fairly well (although they do lose some of the fine detail over time)