If you've randomly stumbled upon this post, go to the "How do I get Started?" page and read the link that explains the Curly Girl Method. The Curly Girl Method is a great way to help dry hair.
Cowashing adds moisture to hair. You will probably want to cowash most of the time.
A moisturizing conditioner
Find a good moisturizing rinse out conditioner that works for your hair. If your hair likes oils, try to find one with oils. Coarse hair tends to like oils. Oils soften coarse hair and make it more bendy. If you have fine hair, you will have to be more cautious about oils. Oils can make fine hair heavy or oily. You will have to experiment to find which oils work for your fine hair (if any). You can mix oils into your conditioner to make your conditioner more moisturizing. Some examples of moisturizing conditioners are: Renpure Organics (red bottle), Biolage, GVP Conditioning Balm (Sally's, generic Biolage), Aubrey Organics Honeysuckle and Rose (oil heavy, strong scent), Darcy's Botanical's Pumpkin Conditioner (lots of oils, you can buy it here), Shea Moisture Restorative Conditioner (Target and Walgreens, has a lot of shea butter, oils, and glycerin if those are issues for you).
Personally, I tend to prefer conditioners with behentrimonium methosulfate. Behentrimonium methosulfate is an excellent moisturizing ingredient. This post from The Natural Haven explains why it is a good moisturizing ingredient. Behentrimonium methosulfate also adds a lot of slip.
Deep Treatments (DTs) help to add moisture to hair. You want to look for a DT with good moisturizing ingredients. Some examples of good ingredients to look for are coconut oil, olive oil, avocado oil, shea butter, jojoba oil, and aloe. If your DT contains things that can penetrate the hair's cortex (like coconut, olive, and avocado oil), leaving a DT on your hair longer then 5 minutes will make it work better. The longer a DT is on your hair, the more it will penetrate into the hair's cortex. In this popular curltalk thread, the author of the thread suggests towel drying your hair (so you don't dilute the DT) and leaving it on your hair for an hour. Towel drying first also helps you use less product. Many experts suggest adding heat to help a DT penetrate into your hair's cortex. Heat opens up the cuticle and allows the DT to penetrate. You should DT as often as necessary, but once a week is common. You can make your own DT by adding things to your rinse out conditioner. You can add oils, aloe, warmed honey, etc. Two popular DTs found online are Curl Junkie Repair Me and Spiral Solutions Deeply Decadent Moisture. Other DTs include One n' Only Argan Hydrating Mask (Sally Beauty Supply), DermOrganic Masque Conditioner (beauty supply stores).
You should use one if you have dry hair. You can use your rinse out conditioner as a leave in.
In low dews, you should seal your hair to keep the moisture in your hair. After applying your leave in conditioner, smooth a few drops of oil over your hair. You can use any oil you wish to seal your hair. Some examples are olive oil and coconut oil. If you have fine hair, you may prefer a lighter oil. Grape seed oil (found by cooking oils) is a light oil and a good one to try. Some people find grape seed oil to be drying. It might help to apply oil to one section of your hair at a time. Other oils to try are sweet almond oil (health food stores) and apricot kernal oil.
The spritz and condish method
This method was developed for dealing with porous hair, but also works for dry hair. Here is the link for a better description. For this method, you spray your hair with water until damp. Since your hair is less wet then it would be if you wet it under running water, your hair soaks up less water and more conditioner.. This will help get your hair moisturized. My cliff notes version of the Spritz and Condish method is: Spritz your hair with water until damp. Add conditioner. Wait 5-10 minutes. (I'd suggest 15 minutes. I've read that after 15 minutes your hair stops soaking up water. If your hair soaks up less water, you will have less hygral fatigue.) Get in the shower and rinse. Cowash or low poo. Add some conditioner to detangle. Style as usual.
What is hygral fatigue? When you stretch a rubber band it returns to its original shape. If you stretch it over and over it no longer returns to its original shape. When hair gets wet, it gets heavier and longer. When it dries it weighs less and is shorter. Like the rubber band, hair can only be stretched so many times before it no longer returns to its original shape. When hair no longer returns to its original shape, that is hygral fatigue. This The Natural Haven post explains hygral fatigue better. I learned about hygral fatigue from that blog.
The Squish 2 Condish Method
This is a method developed by curly hair stylist Melissa Stites. This link from her blog explains how to do the method in depth. In short, it is a method of rinsing out your conditioner while squishing in water. This will leave your hair with the amount of conditioner it needs as a leave in conditioner when you are finished.
The LOC Method
LOC stands for Liquid/Oil/Cream. Basically, after you rinse out your conditioner, you 1) add a leave-in 2) seal your hair with a few drops of oil 3) add a cream (like a curl cream). See this Curly Nikki link for more info.
This post on Science-y Hair Blog discusses which oils penetrate the hair's cuticle. Coconut oil, sunflower oil, olive oil, and argan oil are all good choices. You can add oils to your DTs or conditioners to make them more moisturizing. Jojoba oil (technically a wax) is very similar to your hair's own oils. You can also add oils to homemade products like flax seed gel or the gelatin protein treatment.
Please read my coconut oil post for more complete information on coconut oil. Coconut oil is an excellent oil for dry hair. You can try using it to scrunch out the crunch. You may be able to add a very small amount to your ends at night. It works great as a prewash treatment. Please do a strand/small section test when you try a prewash treatment the first time. Coconut oil is hard to remove for some hair and very easy to remove for others (me). If it is hard to remove for you hair, you wouldn't want your whole head to end up oily the next day.
If your hair is porous, you need to close the holes in your hair's cuticle so the moisture you are diligently adding doesn't just escape again through the holes. Protein closes the holes in the cuticle and locks the moisture in. Be sure to read the links on protein (link 1, link 2). Protein is not for all hair and can be bad for some hair types. If you have determined your hair should have protein, you should experiment with different kinds of protein until you find the one that works best for you. To learn how finding the right protein made a difference in my hair, read My Gelatin Protein Treatment Testimonial in this link. The difference was night and day. Getting the right kind of protein for my hair was the single biggest step for getting my hair properly moisturized. The magic protein for my hair was collagen. For your hair it may be another protein. Be sure to test both collagen and keratin when searching for a protein that works for your hair.
Honey doesn't have a high success rate for dealing with dry hair. But, when I had very dry hair, my hair absolutely loved it. My picky hair rejected most of the ingredients that make a conditioner moisturizing. I was having a terrible time finding a conditioner moisturizing enough for my hair. I started to mix honey into my rinse out conditioner and it worked like magic. I mixed 50% warmed honey and 50% conditioner to use as my rinse out. I mixed it up each time I used my conditioner. If you mix honey directly into your bottle, you will grow creepy crawlies (technical term) in your bottle. The preservative in your conditioner will not be able to handle the addition of honey. I also mix honey into my gelatin PT.
Curl Enhancing Jelly Products
Curl enhancing products containing aloe or homemade flax seed gel add some moisture to hair. Every little bit helps. A curl cream would also be a good choice. Some examples of aloe containing curl enhancers are Kinky Curly Curling Custard (Target, online), Spiral Solutions Curl Enhancing Jelly, Spiral Solutions Firm Hold Gel (online), Curl Junkie Coffee Coco Curl Cream (online). You can buy flax seed gel or okra gel at SweetCurlElixirs on Etsy or you can make them yourself.
Curl Creams: Kinky Curly Knot Today (Target, Whole Foods, legendary for detangling), Bioterra Curl Cream (Sally's), and Shea Moisture Curl Enhancing Smoothie (Target ,Walgreens, Sally's).
Which ingredients to watch out for
Humectants in low dews can dry your hair. Humectants draw moisture to themselves. In normal dew points, they draw moisture from the air into your hair. In low dew points (under 45-30, depending on your hair), there is not enough moisture in the air for humectants to draw moisture from the air. Humectants have a strong pull and must get their moisture from somewhere. In low dews, humectants pull moisture from your hair. The humectant people seem to have the biggest problems with is glycerin. Glycerin is the biggest problem in things left on your hair, such as leave in conditioner and styling products. See my post on Humectants and Dew Points. Magnesium sulfate (Epsom salt) can be drying for hair. It is added to a lot of products for curly hair because it is curl enhancing. If humectants or magnesium sulfate don't dry out your hair, they are fine for you to use. Everyone's hair is different. The rules don't apply to everyone.
Thanks to everyone in the Wavy Hair Community that helped suggest products for this post.