In his talk on creativity, John Cleese says you should give yourself about an hour and a half of time to play with ideas — the first half-hour to calm your thoughts down and the next hour to actually have ideas.
I've noticed that I tend to suddenly get ideas when I take long walks. It's hard to use a smartphone when you're walking, which is good for you, since you have to invent your own thoughts instead of consuming other people's thoughts.