How do fashion marketers keep up with the new and different fashion trends in everyday society?

A good marketer has a strong passion & natural curiosity for their industry. They likely have been interested in that industry all their lives, so they always want to stay on top of the newest trends. There are 'formal' and 'informal' ways to stay relevant, and a good marketer should do both:

More formal / structured -- consistently read news, blogs and magazines that are about your industry. For fashion / beauty, some of my favorites are:
- Women's Wear Daily: daily newspaper that is one of the leading sources of fashion-related news. Ask for a subscription as a gift!

- Streetstyle blogs: The Sartorialist, ManRepeller, WhoWhatWear
- Marketing sources like Ad Age

More informal -- this feels less like work, more like fun. Identify brands & retailers that inspire you, and visit their stores every few months (at least). Being out in the market is absolutely the best way to stay relevant.

This is a similar response I posted in a fashion question early today- but still relates to your question as well.

The fashion industry just as technology is in constant flux. But new ideas are generated from creative ideas of inspiration from those all around you and your visual world. Something that I find amazing with fashion but enhanced with technology is wearable technology- this is a big piece of the next visioning stage with the fashion industry and fashion accent wearable pieces.

Here are some amazing fashion design ideas to add to your inspiration digital tool box today:
Intel and Wearable Tech http://www.intel.com/content/www/us/en/wearables/wearables-overview.html
Fashion week with wearable tech http://www.wareable.com/fashion/new-york-fashion-week-wearable-tech-september-2015
Future Fashion http://www.cnbc.com/2013/06/28/Future-Fashion:-10-Wearable-Tech-Trends-to-Watch.html