On the computer. It works for me since most of the ideas I want to save come from online in the first place. So I just bookmark them into various folders (sewing, paper crafts, recipes, etc.).
I do come across the odd magazine page or something I want to save, and I'll just scan it and save it in the appropriate folder on my computer.
Or, if it's really simple and just a few short phrases where scanning would actually be more time consuming, I'll just type it into a Word document to save it. I have a Word document going about plans for DS's birthday party right now, with links to ideas from the web that I've liked.