I received a question this week from a reader about cleaning her doll's hair, so I thought I would address this issue in an article. The short answer to how you should clean your doll's hair is: it depends!
This is a case where an ounce of prevention is worth a ton of cure. The best way to prevent needing to clean doll hair is to keep them in a case to prevent dust and cooking grease, smoke, etc. from gathering on your dolls in the first place. But, assuming you haven't done that and your doll needs some hair care, here's my best advice.
If you have an antique, or vintage, doll you should not wash it, for several reasons. The wig on your doll's head is probably glued on and you never want to get the glue wet. Also, you don't know how the hair was styled and getting it wet is sure to ruin the style. Your best course of action with antique and vintage dolls is to just dust the hair. This can be done with a can of compressed air, such as you would use to blow dust off your computer keyboard. This can also be done with a hairdryer but you might want to use the diffuser to avoid blowing out the style.
For dolls with rooted hair,such as American Girl or Magic Attic type dolls, you can wash and style their hair. Keep in mind that a doll's hair is not human hair and you don't use your hair products on it. If the hair looks dull or dirty, gently comb it out. Then wash it with a small amount of wig cleaner and cool water and rinse thoroughly. You will want to keep the doll out of the water as much as possible to avoid damaging the face paint.
If the hair seems clean but frizzy or tangled and the style is unruly then a conditioning treatment with fabric softener. Yes, you read that right! This is known in doll collecting circles as a "downy dunk." You pour about an inch of liquid fabric softener into a basin and put the doll hair in it to soak. Keep all the doll's vinyl parts out of the basin, but get the hair as saturated as possible. Let the hair soak in the fabric softener for about an hour, then rinse thoroughly and allow the hair to air dry. Once the hair is dry you can re-style the hair as you wish.