As well as you know creating a polished user interface is an important part of a successful application. Although the Flex has powerful default skin and looks not bad. But from the business point of view you should consider more about the comany’s identity,unique, the difference from competitors and need meet the target customer’s sense. Then you need to be an ability to customize the look for your own. No matter you are just a beginner or an expertise, we all recommand these tutorials to read. Athough some of them seems very old by now written in 2006, but still very classical.  Okay,let’s enjoy!

Flex 2.0.1 - Understand Runtime CSS ( by Ted Patrick ) 
It introduced Runtime CSS example in Flex 2.0.1.Now your expecting CSS to be text based but in Flex’s case to support custom fonts, custom component skins, and graphics embedding the runtime format of CSS in Flex is SWF. Through it we could learn how to set up CSS in runtime.

Interface Customization in Adobe Flex ( by Tariq Ahmed )
It introduced Adobe Flex Interface Customization:the basics of Themes, Styles and Skins.
One of the great things about Flex is that you spend most of your time working on the business logic and workflow, and less on the cosmetic aspect. It is because the Flex default’s look-and-feel is great! The only problem of course is that most people won’t spend much time customizing the look, resulting in many Flex apps looking the same. But if you want your website application has own unique charming, you’d better customize in Adobe Flex by yourself.

Flex Quick Starts: Building an advanced user interface ( By Aral Balkan )
It is a basic tutorial published in Adobe Official Website which help you to quickly learn how to build an advanced user interface. 

Adobe Live Docs: Using Skins (N/A)
It is another tutorial published in Adobe Official Website. This topic describes how to add skins to Adobe Flex components by using ActionScript class files or image files. 

Skinning and theme support  ( by Glenn Ruehle, PDF file ) 
If you want to know the details of skinning and theme support in Flex 2 there is no better guide than Glenn Ruehle. Here are Glenn’s slides from his technical session on Skinning and Theme support at the Adobe Component Developers Summit.

Designing Flex 2 skins with Flash, Photoshop, or Fireworks ( by Narciso Jaramillo )
This article describes how to use Flash, Photoshop, Fireworks, or Illustrator to create a set of skins for the Flex component set. Using the skin template files provided with this article, you can draw your new skins, then go through a simple export process to create the skin asset files that Flex needs.

Designing Flex 3 skins and styles using Creative Suite 3 and Flex Builder 3 ( by Narciso Jaramillo )
This article provides an overview of the CS3 extensions and Flex Builder 3 features that automate the skinning and styling workflow.

Programmatic Skinning with Flex 2 ( by Kevin Hoyt ) 
It is also an OLD article written in Feb.2006. This article teach you how to create a programmatic skin in Flex 2.

Adobe Flex 2 Example: Skinning Framework support for 9-slice MovieClips from Adobe Flash Professional 8 ( by Waldo Smeets )
It is also an OLD article written in Feb.2006. It is supply the demo and source code download for the example which process of making your own expressive designs for the Adobe Flex Framework, through skins, has been dramatically improved in the new framework. 

Presentation on skinning and styling ( by Andrew Trice )
It is a very OLD article written in Dec.2006. You will download some demo source code.

Flex Skinning: Introduction w/ Buttons (Part 1) ( by John Blanco Littleton )
It is a very interesting articl and explain how to style, skin, and customize your Flex components in a humor sense. At least you won’t fell boring.

Flex Skinning: Programmatic Skins (Part 2) ( by John Blanco Littleton )
This time the author reviewed Button skinning and then launch into a discussion about how to skin using code rather than graphics.

Documentation for the Flex Skin Design Extensions
This article describes how to create skins for Adobe Flex applications by using Adobe Flash CS3, Adobe Illustrator CS3, Adobe Photoshop CS3, and Adobe Fireworks CS3, and then import the skins into a Flex Builder project.

How to Skin a Flex Video Player ( by Dolores Joya )
This is a small video tutorials for creating Flex skins from photoshop.It shows all details from “Create a New Flex Project” to “Test Project Turn Off the Scrollbars for the Flex Component”(see details by click above link).I think this tutorial is very easily understand for EVERYONE.

Flex Developer Center: Design
It is not a tutorial but a community for Flex Developer. You will get much resource of UI design—Include shared insights into the darkest corners of the Flex UI design from Experts.