I tend to have three copies: a copy of everything, a backup copy of everything, and then the selected ones. It is a bit ridiculous, but I have found, as Tibor said, that sometimes I want the image I deleted or thought I’d never use. So, for example, overexposed images can be just right for some graphic element of a website…. What really takes a huge amount of time is going through the images, getting rid of the really unnecessary ones, and also organising the others in some way that I will find them again (my curent favorite is to organise them by potential use – exhibitions, greeting cards, websites). It can be very time-consuming to find that picture that you know was fabulous, if only oyu knew when and where you took it.
When I can, I’ve started being choosier about my shots — someone recently criticised me in fact for only shooting when I was confident the picture might be interesting. He said, ‘you’re not using film! It doesn’t cost!’ but that’s not at all a useful approach (I also have three of four boxes in the basement of slides and negatives.)