If you've purchased an area rug online or in a store, you've gotten the upsell pitch to buy the padding to go underneath. If the rug is thick enough that it's not an issue of sound barrier but you would like it to stay in place, this is a cheap and easy solution (thank you Martha Stewart).
I just did it myself on both my new area rugs and I can attest to the fact that it does indeed work wonderfully. Not the best photo (below) since the back of the rug is white, but you'll get the idea from Martha's photo.
As you can see, I left some blobs in places and was not exact in the distance between lines. It really doesn't matter as long as you get enough on there throughout. Let it dry overnight to be sure it's ready to flip. You can even buy caulk that is clear. You'll need less than a tube of caulk which is only $2.73 and the 'gun' to squirt it out with is only $1.97. It's super easy to use and it saves you $20-30 for each rug you would have bought a pad for. BONUS: the caulk doesn't dry up so you can put it away with the gun and use it for years to come.
Sneak peek: the chair in this photo is my next project. It had an insufficient amount of padding on the seat and a super old fabric that had disintegrated over the years. I bought cool new fabric, some batting and have my trusty stapler on hand to finish the job. Wait to see what I accomplish in my next post (fingers crossed).