In a spray bottle, mix a small amount of laundry detergent in with fresh hydrogen peroxide, and gently shake it to mix it. Spray on the stain; it will start bubbling up. Scrub and blot with a clean rag or white paper towel. This works for all protein-based stains (blood, pet stains, etc), and even red wine. If you have cats that leave a hairball on the carpet (even white carpet), you can spray the mixture directly on and just wipe it up with a paper towel - no rinsing.
