[ohashi]

It varies. Often times I find myself doing something completely unrelated and an idea will pop into my head. It has even happened while sleeping (I woke up with an idea just begging to be explored).

The only consistent thing I've noticed is that ideas come to me when I am struggling or unhappy. Those are the ideas of the 'this could be improved' variety.

If you aren't coming up even with those, well, you can get outside help for ideas. Try attending a startup weekend. Listen to other people's ideas. What do you like? What don't you like? Why? Could you do it better? Start asking yourself (or even others) questions about them and see where it takes you.