Always buy fresh prawns for your cocktail. A good, fresh prawn will be shiny, unblemished, firm, and have a delicate and non-pungent aroma. If you are not sure of their freshness, use frozen ones. Another tip: before you eat your prawn cocktail let it rest for at least half an hour in the fridge, and if you don’t have time to make the cocktail sauce you can buy some ready-made at the supermarket. Finally, remember that the cocktail sauce must be creamy and thick, not runny.