Where do ideas come from?
They're all around you, but you have to hunt them down and capture them. You need to be a:
Spy: Listen to other people talking. This is great for getting the sounds and rhythms of dialogue. You can also get some fantastic phrases to use - especially when you don't quite hear correctly what the other person said! My favourite mis-heard phrases are 'The doom is in the middle of the yard.' and 'The shadow men are coming for me.' (Not things that people are really likely to having been saying on the bus home from work!)
Thief or maybe even a Vampire: Steal things, or even bits of people (well, not actually...) I have a collection of names waiting for characters to claim them, that I've stolen from real people. Suck characteristics out of people: a laugh, a habit or mannerism, a way of talking... it's these details that will make your characters come to life. You don't have to invent everything in a piece of fiction. You could steal a setting - I stole the whole of Edinburgh!
Cannibal: If something doesn't work in one piece of writing, don't throw it out. Cannibalise it and use the bits you really like in something else.
Some useful habits...
Keep a notebook. It's harder to lose than lots of scraps of paper (I speak from bitter experience..) Use it to keep a note of ideas, names, jokes, phrases - even single words. You never know when one of them will be exactly what you need.
Find a folder. Keep pictures, articles from newspapers and magazines, postcards, tickets...anything that has sparked your imagination.
Keep a pen and paper beside your bed. Ideas often pop into your head during the night. You know you'll have forgotten them by the morning, and you don't really want to get out of your cosy bed on a cold night to look for something to write with, do you?