The old "how do you come up with ideas" question is an oft asked one, and the answers are always different for everyone.
I get a lot of my ideas from The Idea Warehouse in Frostbite Falls, Minnesota. Cheap prices and free shipping! (For those without the needed background - that is sarcasm! Frostbite Falls, MN is where the main characters in Rocky and Bullwinkle come from).
On a less sarcastic note, you can't force ideas. I find doing something entirely unrelated helps clear the mind so ideas can find their way in all by themselves. Take a walk. Read a comic book. Watch cartoons (Rocky and Bullwinkle works almost as well as Bugs Bunny!) I find just playing with my cats is a tremendous way to get in the right mindset for creative work. The most important thing is to start with as empty a slate as possible, then look over the job guidelines, and just "do without thinking" until something nice appears - then think.
It's more of a "Zen" thing with me than a planned out process. Every job is different, so you can't use the same process for everything. Ideas happen all by themselves if you (I) let them, but try to force them and it's all very subjective in-jokes that only I and a select few of my friends will ever get 