For those making two part molds for helmets this tutorial will help.
Now the tutorial is not on making molds for helmets specificly, but will help give ideas.Dan Perez Studios molding and casting tutorials
If you dont have a pressure pot, just use the air vents ideas mentioned in the tutorial using toothpics glued to small areas to allow air to squeeze out.
Depending on the RTV rubber used, sometimes before pouring the mold, use a old paint brush to make sure small crevaces are filled with mold material and air bubbles on the surface are removed before pouring. I found with my helmet casts this saves alot of hassles and makes sure molds work great first time arround. (This again is for those who dont have a pressure pot.)
Also a technique to use if making latex molds of head casts, make small cuts with an exacto knife in the location of the nose, chin, ears or any other part that might suffer air bubble problems. Make sure it is a small clean cut with a sharp exacto. When pouring casting resin it helps push the air bubbles out, and if its a small incision the casting resin should not leak out. I have been doing this for a while with alot of success.
As someone who has been using Rubber Urethane RTV for the last year, who has now started using silicone rubber, honestly, save the hassle and be patient and get the right stuff for the job. I am a horribly impatient person. I went the route of getting anything I could to start molding and casting right away and it was not only way more expensive in the long run but caused far far more frustration than it was worth. Check out Smooth-Ons distributer links. In many cases there is a company that can ship the stuff faster and cheaper than ordering it directly from the store. (Or the other mold making companies mentioned in this thread.)
Though if you MUST have something right away, another store you can check and see if its in your area is TAP plastics. Its not the greatest, but its out there. Their casting resin sucks for anything other than prototype work.
Well enough of my rambling on the subject...