1. Look at other designs
The no. 1, most important way, and most often cited way of getting an idea is to look at other designs. This may entail going to sites like deviantART and exploring the designs portion of the site. I assure you that you will like more than one thing there. Find the artists that you like and keep track of their works.
2. Get screen capture software
Best way to remember something is to capture it and store it. Capture screens of website designs and videos of animation and/or interactions.
Screen capture software like Debut Video Capture is very handy because it captures stills and videos, but most of all it supports different formats. Video formats like AVI is overkill because you want to capture 3-5 seconds of an interaction and it eats 50-100mb depending on screen size. It also supports recording regions on the screen, which is exactly what you want.
3. Get a camera
A phone camera will suffice because you want to remember what you saw and why you liked it. Architecture, colors and the overall background may reasonate with you and influence your design. Heck, you may even use the photo as a background somewhere.
4. Get a notebook
Not a physical notebook, though you could. I mean something like Evernote or Microsoft OneNote. Why? You can drag your screen stills or videos into them and keep notes on why you like them and remove them later if necessary. Even better is that if you have a tablet/convertible tablet, then you can draw mockups based on those designs that capture what you liked.
Periodically, I would say every 3-6 months, check your collection and remove what you don't like or reasonate with anymore. You will find things that you find boring or out of date. You don't have to remove them at first sight, but you can mentally mark them for deletion.
I find that when I need an idea I go straight to my designs folder and attempt to create a mockup of what I liked. Something good and different usually comes out. I think it is the excitement of looking at what you compiled that drives you to innovate. There isn't much to it. It is like your photo collection of your life. You collect what you like and, more than not, are influenced by it. Explore and create.