I learned what the attachment items were and whats considered an accessory. Then I always tried to interact with a guest trying to learn what they are wanting to buy and figure out what their needs are.
I usually told guests that buying the 'so&so' accessory helps with quality, capacity, or the experience.
If a guest is buying a new camera, or moving from a 5Mp camera to 12Mp camera. They need a memory card with lots of space. Since memory is fairly cheap now, or they love taking pictures they will need a high capacity card. And its better to use rechargeable batteries because you don't have to buy batteries all the time.
Printers. They only come with a sample cartridge and no USB cable. So suggest to them they will also need a USB cable and color/B&W cartridges.
Game systems. Always ask what games they like or like to play, or point out some new releases. If they like 2 or more player games, tell they need another controller.
If they are getting a Wii, show them all the different controller accessories and they help play the games easier.
DVD players or TVs. Always hype up how HDMI cables give better picture quality than using coaxial cables.