HEy Paul,
When I first started working obsidian I was having the same problems. You really don't need to hit obsidian nearly as hard as tougher materials like chert. I kept hitting it too hard, sending overshot flakes that would go too far and break out edges on the other side. I would also try to knock spalls off a big piece, and I would hit it so hard that the spall would break apart into 2 or 3 pieces. If you're trying to knock large spalls off a big block, you really need to grind your edges very heavily in order to strengthen it so it won't shatter when you hit it. And don't hit it as hard. Obsidian breaks very easily, so if your edges are set up properly it really doesn't take much force to knock off very large flakes. Try easing up on the amount of force you apply to it. I had been used to knapping tough chert, and was using the same technique on obsidian, which turned out to be too much. When I started tapping it instead of really slamming it, my results improved considerably.
Also, I found that using hammerstones of medium softness can help too, because they don't send such long flakes.
But if you are new to knapping and don't have anyone to guide you, there probably is a good chance that you're angles are wrong or your platforms aren't being set up properly....that could be a problem as well. If you're brand new to knapping, my suggestion is to try pressure chipping arrow heads out of the thick, flat bottoms of large beer and wine bottles. That way you can practice and don't have to worry so much about ruining obsidian that you paid money for.
