I spend a large amount of my time travelling on trains and tubes, between east and west. So I guess that's a situation where I get a lot of ideas and times to think things through. I go through quite a lot of sketchbooks, I think its important to write as well as draw your ideas.
Also I feel like I get a lot of ideas in situations when you can't really say your own ideas because other people are in charge. For instance when I was interning at the art department at ELLE, I came up with so many ideas in a day because I was thinking of ways they could improve what they do but couldn't voice it because it would be unproffessional, but I can apply it in my own practice - if that makes sense!