Back when advertising folk all looked like Cary Grant, James Webb Young penned a short classic: 'A Technique for Producing Ideas'. Download it here.
In a nutshell then,
Step 1: Leave no stone unturned : Do all the homework on your current brief and - on an ongoing basis - browse the world for general, random 'interseting' stuff.
Step 2: The Mental Digestive Process. Chew on what you've learnt.
Step 3: Now drop it completely. Have a bath. Do something else.
Step 4: Eueka, the idea appears.
Step 5: Share the idea. Work it. Develop it.