you need to be responsive to what's happening around you (i.e. the evolving culture), or as paul priday put it 'where consumers do what they need to do, to get what they want'.
Being 'creative' is a term also to be wary of. You can't just 'be creative'. In marketing being creative isn't about being able to come up with something original. We need to be accountable for our ideas. Our creativity is for hire, thus our creativity must provide RESULTS. Sadly, not many people are able to do this. I have been in groups with very creative individuals who simply don't realise their ideas won't work.
Another observation of mine is that advertising and marketing cross over a lot, and even in brand management positions, eventually a culmination of your efforts will likely be an ad (albeit through an ad agency). Thus, you must really love advertisements. If you can look at some ads and truly love and idolise them: i.e. get soooo much pleasure from them, then perhaps you're in the correct industry (and advertising may even be a better idea for you, than marketing-based positions).