help please...crevettes....

when we were at aqua grill last month, i had the crevettes off the bar menu. they came dripping in garlic butter and sprinkled with parsley. to die for. they were tasty, finger-lickingly delicious and we each ate a pound just as a starter. yes, each of us, a pound each. they were that good. the thing is, we've tried to recreate this seemingly simple dish at home, and well, failed. how does one get the shrimp shells so soft that they're edible and the crevette heads fall off with just one poke??? do i boil or steam the prawns first? what about overcooking them? i steamed them for 5 mins then tossed them in garlic and oil...but still the shells were not soft enough. is it dependent on the age of the shrimp? help help help...