Keep a regular notebook of ideas you have seen and drawn out by hand with a pencil. This is also great for card designs as well as scrapbook pages.

If you don't like the notebook, use index cards and a file box.

When you see an idea, just sketch it out into your notebook or on your index card. You can write down additional information, like where you saw it, colors, stamps, and maybe how you might use the idea.

Bring cards or the notebook when you go to the stamp shop or a convention.

Keep near your stamp/scrap space.

When you need inspiration check it out.