I also work in advertising and face the same challenge - maybe more so as I'm 50. I've found a couple of ways that work for me - but I'm definitely open to hearing other suggestions and experiences.
One approach I have is to wear classic clothing, but with a twist. For example, color: I have a pair of bottle green trousers, very slim cut. Paired with a sports coat, FC shirt etc, it makes a classic look but the color shows you're using your imagination. Another option is to mix in a single item that's not so classic. You have to be careful with this, but for example I frequently wear FC shirts - sometimes solids or classic patterns, sometimes bolder - with jeans. Or dress pants in say POW check with a dress shirt, but with a jeans jacket. It's not for everyone, but it shows some spirit. Also, in my office, my colleagues only wear suits for important meetings, but I occasionally wear a suit just because "I feel like it"... adds an element of surprise. One other opportunity is the bow tie... I use this a lot, again, to add an unexpected note.
The aim is to show creativity - that you're wearing classic clothing, not because you have no imagination, but because you DO - that it's your way of being creative without resorting to the clichÃ© of graphic T, jeans and Converse. Someone on this board has a signature line "I dress to express, not to impress," and I think that's the key.
I know some of these suggestions are SF heresy, but they work for me personally, they work for my environment, and they work for the business image I need to project.