For me, I find that the more creative people and ideas with which I surround myself, the more I'm able to think outside of the proverbial box. Beyond that's it's the discipline of listening to the ideas that I have and allowing myself to explore them instead of listening to the negative self-talk of "oh, that idea sucks" or "no one's going to like that".
A big part of that is being in an environment that nurtures creative thinking and allows the flexibility and encouragement to do so. Without that, you'll need to develop creative ways to put yourself into a mindset that nurtures your creativity. That may involve getting involved in some more creative activities (writing, painting, photography) outside of the workplace, and letting that style of thinking carry over to the workplace.
I am a contractor who works in media. While I have experience in film production, the marketplace wants web developers right now, so I use my technical background to manage web media and content. The creative and the mundane are always battling, but because I maintain a balance, I've always got a creative idea lurking somewhere.
I answered a question about creativity here a while back; that may include some ideas and tools for you as well.