Great question. I think the big thing is that you have to get exposed to different things. If you don't have your own itch that could be scratched for profit, then you need to find somebody else's. Some of the posters mentioned learning about different domains and niches; to me that's the key. If you are a problem solver with an entrepreneurial streak, you will find itches that need to get scratched just from talking to people about what they do.
The key is: how do you get exposed to new domains? I'm sure most business managers/owners could name 3 or 4 things they wish they could make better. However starting that conversation with them is difficult.
My wife does some direct sales stuff (Pampered Chef, Lia Sophia) and the key concept with their sales training is: get on the damn phone! Make 3 calls a night, that's it and your business will grow.
Idea generation is the same thing. Just think if you could have 3 conversations a day (heck, even 1 conversation a day) with people about what they do during the day. I bet you'd have multiple actionable ideas in a week.
It sounds easy and trivial, but it isn't. I know this works but I haven't done it yet. I'm not brave enough, charismatic enough, too big of a p*y to just start up conversations with people, at least so far... But I think if you go to the core of most business startups, that didn't start with a guy/girl solving her own problem, you'll find that most of them started by putting themselves out there.