You've probably lain in the grass and looked at the clouds before. One can recognize many things, many images or elements in the clouds with a little phantasy.

This way of playing with your imagination is a good method to train and boost your creativity.

Of course, this method is not limited to clouds. You can use any form of irregular pattern, for example marble, wood grain, crumpled paper, fabric wrinkles etc.

It may also be helpful to start an image album, similar to the idea book. You can store all the more-or-less useful images from all kinds of topics.

If you don't have an idea some time, you can take this album and leaf through it - maybe one image or another will provide you with a new idea.