How do I prepare prawns such that they are straight after boiling/steaming?
Wash the shrimp and remove only the head. 
Insert a bamboo skewer along the shrimp from head to tail, running along the legs without touching flesh
Drop into boiling, salted water for 3-5 minutes (do not put a lid on); after this period, they'll change color and rise to the top. If they feel firm, they are cooked. 
Quickly place them into ice water, which helps them have good color and stops the flesh from shrinking and becoming hard. 
When shrimp are cold, remove from ice water and drain in a colander. 
To remove the skewer, use a screwing motion.


For nigiri-sushi: 


Remove shell from around body, but not the tail.
Butterfly cut open, cutting from head to tail along the belly with the knife only going halfway in. 
Use the knife or your fingers to open it up and flatten it carefully, without further breaking the flesh. 
Remove vein and rinse with lightly salted water. Lay on papertowels to drain. 


If using for sushi rolls, remove tail and cut in half lengthwise.