What is the best thing to do when I cannot get design ideas?

Take a quick trip to local art galleries or browse through art books. Maybe that can get the juices flowing again. If that doesn't work, you may need to just take a break from it for a while- Go fishing, visit a night club, have a beer if you drink. Relax for a bit. Good luck.

relax, clear your mind, do something else and then come back to thinking of designs, listen to music, imagine who the you're designing for and who will see it and try to think of what they might like.

to stimulate the creative mind, one must do something that has no relevance to the project. Then, creativity will flow as ALL things are relative.

You need to try not to limit yourself mentally. Walk around, talk to friends, ride a bike, do what ever you need to do to become inspired. Remember the things that lie closest to our mind and soul is what inspires us the most and that can be seen in the work we do.

Music is one the most powerful catalyst for creative inspiration. Not just any music though. The best music to listen to is something dynamic and nonabrasive, usually instrumental. Just let it play in the background while you take advantage of the techniques offered above and below this answer.

I study fashion design, when i have that problem i try to relax, nature is the key for me. try to see more than you see, look the shapes, the colors, the texture ( for example, the sand, your carpet at home, flowers, the contrast of light and shadow) Believe silly things you have around you can help you a lot. In my case take a walk, look at people, always works. play with illustrator or photoshop or your design program, everything will come to your head