One of the challenges of being a designer is the need to produce creative work on demand. No matter how heavy your workload, every client deserves your best efforts--after all, that's why they hired you. In personal work, we have the luxury of being able to wait for ideas, and that's really the stereotype of the typical artist, someone who goes through intense productive spurts, and then periods of rest. Well, as designers, we're not exactly artists, and probably don't get much rest!
Here are some ideas to jumpstart creativity on those days when you're just not feeling it :)
Do a Time Challenge
Setting a time challenge is a good way to start off the design process. When I do this, I try to produce a bunch of rough layouts in a given timeframe. I like to go for 1 hour, and I usually do these in grayscale and without logos, to get down to just the basic shapes and composition. When you're moving quickly, not focusing on details, and not committing to any one idea, your mind is more free. Creativity should follow.
Change Your Location
In your home or office, you're comfortable. You know how the day is going to progress, and there are distractions you can expect. These are all impediments to getting new ideas. Break the mold by taking your laptop to the coffee shop or library. The distractions are different and the pattern is unfamiliar, and that can give birth to a new idea.
Change Your Media
Sometimes I can get fresh ideas by using tools that are out of the ordinary for me. If you usually design in Photoshop, switch to Illustrator, or vice versa. Or go to paper sketches and/or cutouts. This can have a bigger effect than you might think.
Take a Break
This comes in handy once you get down to the detail stage of a design, and it can work miracles. You need to get your mind off your problem for a little while--overnight if possible, but at least 10 minutes. When you return, you'll be less frustrated and rushed, and you can think about the problem in new ways. There have been numerous times when I've slaved away for hours at ActionScript errors that seemed impossible. When I woke up the next morning, I immediately came up with an idea about what the cause could be, got to work, and solved it.
Conclusion
In my experience, you can give your creative mind a little bump by doing pretty much anything that breaks out of normal patterns. Ideas of your own? Please comment, I'd love to hear them.