There are many different varieties, cooked and uncooked (green), frozen, large and small.
The most important thing is that the prawns are fresh. Fresh, uncooked is best. Look for fresh green prawns with a fresh smell and no hint of black oozing juices or blackness around the head. If they are unavailable buy frozen green prawns.
Buy cooked prawns direct from the market and check that they've been cooked the same day.
Cook and eat them the day of purchase, or freeze green prawns for up to 3 months.
Tips & Tricks:
Never buy cooked prawns and re-heat them.
1 kg prawns yields approx 500g prawn meat. (Source: Stephanie Alexander's Cooks Companion.)
To boil green prawns: Remove the shells and devein them. Drop the prawns into well-salted boiling water bring the water back to boil quickly before reducing to simmer to cook for 3 - 4 minutes.