Concept is Key!

First, hold an open-ended brainstorming session. Anything goes.
Keep track of all potential elements that relate to your goals. Write, sketch. Any detail could be important later.
Keep an open mind. Good designers are fascinated with all forms of communication: music, architecture, photography, theater, film, etc.
Don't judge too quickly. Some ideas must be refined or re-shaped but are good starts.
Narrow down the ideas, balancing intuition with pragmatism. Does the idea serve the goals? Learn to articulate why an idea does or doesn't work.

Teams of writers, designers and...?

Two heads - or more - can be better than one... but 'design by committee' can stunt the process, especially if the group is overly hierarchical. It's best if everyone feels free to suggest a bad idea.
Writers often suggest brilliant graphics, and vice versa. Evaluate all ideas on their own merit - not on who suggested them.
Clients, inside or outside a company, often have good ideas. Listen, but don't give in to a bad idea if you can help it.

Humor is Surprise. We love to discover a unique image or combination of word and picture that we don't expect.
Emotion: don't leave it out. Successful businesses project an image focused on human beings - not just the bottom line
Nostalgia. Creativity often means recycling old ideas by recombining them in fresh new ways.
Pay attention to tone. Does the idea 'feel' the way you want the viewer to feel?

The element of chance can be a much-needed breath of fresh air. Some designers believe that too much goal-setting and noodling of ideas kills that precious moment of sheer inspiration. You'll know when it happens to you.
Sometimes what you want to do is break the rules, even ones that seem to make sense, such as legible text. Many designers have produced fine work by challenging preconceptions and rules.

next: Working with visual elements