I saw several pictures of professionals jamming in a room covered with egg carton shaped like foam. Makes you wonder the egg carton business might work. I also wanted to soundproof my garage a while ago, and I came to the conclusion with "Batts" (hope that's not a aussie word) but these look like big bricks of foam which absorbs sound. You install these in between walls and be hidden so your garage doesn't look crap. You can get these from hardware stores. Well thats my plan when we build our new double garage.

The last resort would be to call in the pros to do it for you but this will cost a fair bit.