One big suggestion is to start with layers and create at least 3 levels of hair color: a dark base, a medium middle, and light highlights. Search the web for hair-care products for examples of the range of colors found in one person's hair and see where highlights should be applied. My last suggestion... use a less blocky brush. You have the right idea with multiple, thin strands, but it needs to... I donno... flow more, is the best I can describe it. Good luck!

The best advice I can give is look at pictures of real people and learn what makes hair look real. Also if you're using Photoshop ( or any similar program that has brush tools that allow this) set you're brush to "fade in X steps". It may be 20 steps, 50 steps you'll know when it's right. Also Tygercub's suggestion about layers is the way to go. When you have a layer that you're happy with stop. Make a new layer and work over the layer you like. You can always merge them together later. Toxicangel has a great tutorial on his site about doing hair as do other's I'm sure. Also Bloodsong's site has some brushes ( furry and hairy I believe they're called ) that you can download. They will work with Photoshop and may work with other graphic's programs. A trick I like to use is when I have painted a section of hair I like on a layer I copy it to another layer and then use it again on another section of the head if I can. They're are so many ways to do hair that I could go on and on. The more you do it the more techniques you'll learn or invent yourself and the better you'll get at it. One last thing is opacity. You can work with a brush that's less than 100% opacity and build up on it. That may give a more "natural" result too.

also the Dodge tool is very useful for drawing in highlights because it lightens the pixels you're drawing over maintaining detail. I usually make a copy of the layer I want to use the dodge tool on so I can go back to an original if I need to.

To help with the blockiness it is useful sometimes to work on a larger size render and reduce afterwards. Paintshop Pro has a smart resize that antialiases when resizing and keeps a lot of the detail when reducing. Also, as with any artist, some reference material helps no end, even to the point of scanning a photo in and pasting some of it into the picture, as per droyd`s copy-and-paste idea.

This tutorial really help me a lot. http://www.adobe.com/print/tips/phsdigitalhair/main.html Diegos