Professional speaker Anne Miller suggests there are different sources for metaphors:
Sourcing,
- Images - "Our data center is a solid brick wall"
- Living and breathing (person or animal) - "This antivirus program is like a snarling doberman"
- Familiar product - "We are the Porsche of supermarkets"
Sourcing,
Tim Wilson
Professional Speech Coach
Free speaking tips at: http://speakingquicktips.blogspot.com