I usually get ideas when either on the toilet, eating, being really busy with something (almost) totally unrelated to games and the idea that comes up itself, or when asleep. (Seriously, it strikes me, wakes me up and keeps me awake for almost the rest of the night).
Some ideas are completely random, others are actually kinda usable perhaps? However my main problem is that I do not have enough time to implement even one of them, and somehow when I think I have time and start planning on implementing/designing that idea I somehow manage to not have time for it (or others do).
I guess my main reason for having the ideas is because I study information technology and it's part of my life, hence the time troubles.
I also guess everyone can have some sort of an own method for finding very specific ideas when looking for them. Mine for example is either playing a very complicated game, or going out for a walk, look at the nature, think about patterns and listening to music (lot's and lot's of music).
Oh and I would almost forget pizza. Pizza is the key component to ideas