This might seem like an obvious one, but never underestimate the laziness of boyfriends (let's face it, it is always boyfriends, isn't it?) when you leave them to do a simple task like keep an eye on the plants or replace the old bouquet with a new one. A surprising amount of bacteria builds up in a vase whilst it plays home to your flowers - to ensure the longest of lives and that the next occupants don't suffer, make sure you remember to give it an actual clean with a long-stemmed brush every three days and before re-filling.