The first thing I do when I am struck for an idea is to start clearing off my stamping table. I almost always find something that jump starts an idea. If I don’t, then I start searching card sketches. When I find one I like, I start making cards. First with scraps, then with my newer papers. Always works for me!
When I am really stuck, I copy (CASE) a card from a set that I really like (from the catalog or something on Pinterest or SUDSOL). By the time I'm finished, I usually have several more ideas to do with that set.
Whenever I'm stuck coming up with an idea I follow this routine: I clear off my desk so I have a clean slate, pick the stamp set/bundle/product I want to use, pick up my Annual Catalog, find a sketch from there that I think will work with the product I want to use an create. I purposely do NOT turn to a device, because you can easily lose 30 min/an hour before you know it looking at Instagram, Pinterest, YouTube, etc. PS, remember when the Annual Catalog used to be called the Idea Book & Catalog years ago? I still call it that...and maybe that's why that's my go-to for inspiration?
I know this wasn't part of the question, but I will also add, when I'm stuck using a new stamp set or bundle, the very first thing I do is stamp every stamp in the set on a piece of cardstock. If there's a coordinating die set or punch, I cut out those images and adhere them to the same piece of cardstock. That way I can "see" everything at once, I know what the images look like, and I can tell what each of the dies do.
Sometimes I take a break for a few days and I stamp for fun so I can relax and not feel the pressure of having to design something for class or blog.
Other times, I've used a sketch to get me going with a stamp I haven't used yet.
At all times, I make sure my desk and room is clean before I start a new project so I can have a clear mind.
When I get stuck and need some inspiration to make a card or more, I first look at my stash of cards I have at home; then I check the Stampin' Up site/Stampin' Connection. I also check the other sites mentioned by others (pinterest, even etsy, splitcoaststampers, etc.) or even do an online search (google, bing, dogpile, etc.). I copy the cards and save them in a folder on my laptop, then go through the folder and put a copy of the cards into an excel table so that I have them all together. Often I combine or mix and match some of the ideas from the cards. I always make a sketch of a card that i think I can make with what supplies I have so I can follow it as well but often end up making a few changes. It's nice to have a collection of the cards in a folder or print the table out so that I have it for future reference too. It helps to have the table to look at for any other similar cards I might have to make as well.