Inspiration can come from anything but it seems that Fashion and graphic design have an especially close relationship. To make sure your designs are "on trend" all you have to do is look to the colours that are appearing on the catwalk and will no doubt filter their way into daily life. I also look to what other illustrators and designers are working on and how they evolve their styles.
One illustrator who's work I absolutely love is Diglee who has illustrated a range of Fashion girls in a range of outfits. They look so amazing she has inspired me to try some fashion illustrations of my own. Ill let you know how I get on.