I picked up Flex really only three weeks ago, read the Flex Training From The Source book and Actionscript 3 with Design Patterns, and started coding. I also picked up The ActionScript 3 Cookbook, which I can also highly recommend.
One of the sites that has been a great inspiration and resource is scale nine, Juan Sanchez' site/blog. Added to the blogroll! Read his information and collections of information on skinning Flex apps.