Well, I promised.
(Though it took a while.)
One of the key ways I know what I want something to look like, is when I'm shopping and I see an item of clothing that is perfect except for ONE, THING. Maybe two things. Knowing how I want to change it urges me to create my own version, but I often forget to come straight home and design it before I forget.
Dress up games are also great for kicking my imagination into gear, especially games that allow many variations and colour palette changes to the items.