The rubber case you use to protect an iPhone needs taken care of from time to time. No matter how clean you are, oil from your hands, dirt and dust will accumulate on the case, signaling that it's time to give it a wash. Manufacturers don't encourage using harsh cleaners on rubber because the chemicals can react with rubber. Clean your case on a regular basis before dust and dirt have a chance to accumulate.
Video of the Day
Remove your iPhone from the case and set it aside.
Add two or three drops of dish soap or mild hand soap to several cups of warm water. Immerse the case in the soapy water, and rub it with a cloth until it is clean.
Use a non-abrasive carpet cleaner to clean the case if soap and water doesn't clean it to your satisfaction. Test a small area on the inside of the rubber case with the cleaner before applying it to the outside of the case to ensure that it doesn't ruin the finish.
Rinse the case thoroughly in warm water. Dry the case with a clean towel and let it air-dry until all of the moisture is gone before putting the iPhone back in the case.