What you need:
white dish cloth
a small glass
Make sure you only use a white dish cloth. The stain will transfer onto the cloth. This is for small everyday stains, not big ones like wine or anything hard to remove. I use this as part of my basic cleaning, when I see a dirty spot on my carpet or couch.
Take the small glass and add a few drops of dish soap. Add water. It should be bubbly. Dip your white cloth in the water and scrub the dirty spot. You may have to do this a few times.
We have really light carpet upstairs and this works amazingly well. Try it out!