I suspect you have them all the time, you just tend not to listen to them.
There's many approaches ...
"Serendipity" ... Random thoughts drop into your head all the time - learn to listen to them. The more you're taking notice of them, the more you'll tend to have. All sorts of odd things trigger - maybe something in the back of the truck you're stuck behind in traffic etc. "Wouldn't it be good if...". Make a note somewhere... always keep a notebook with you if you don't have a smartphone. I've even resorted to calling my own mobile to leave message on answering machine!
"Brainstorming" ... Trusted friend works best for me, maybe you're better with a group... but just bounce daft ideas off each other. Often you can get the germ of something interesting in relatively short time. And the germ of 20 ridiculous things, but hey!
"Scratch your own itch" ... What in your world SW or otherwise would make life easier, more interesting whatever. This may not be the best for a saleable product, but it's worked for plenty. The programming utility market is pretty flooded though.
"Be Open" ... This is a bit like trying to explain being in the zone, but the more you seek ideas, and are open to them, the more ridiculous ideas bubble up into your conscious. Encourage it, think them over and they'll come more frequently.
"Play games" ... Mind games in this case... find 20 things wrong or missing with Facebook / Basecamp, or 20 hates about whatever product you have near / use often ... too easy to get 20? Do 20 more then. When you're really struggling the silly ideas come. When your list is done you possibly have the germ of an addon, API plugin, alternative product.
"Scale / Change" ... What if we had 1000x more bandwidth - what would be interesting? (Realtime 3d video to mobile maybe?) 1000x less, knowing what we do now? (super compression?), 100x more CPU etc... No more yyy available? etc. Many changes will come - that's inevitable with IT. Things like Youtube work due in part to timing of available bandwidth for uploading/downloading - 20 years earlier it's just scifi. Handwriting recognition, RFID, barcodes all had their moment - there's more coming, just not ready for prime time.
Now... you'll throw 95% away, but you'll start to get some that are interesting, some that need thought, some for next year etc. Keep a list.
Don't rule out all the daft straight away - Twitter was a ridiculous idea - seems to have worked though, hey?
Hope that wasn't too vague to be useful. Good luck!