This is paraphrased from Benjamin Graham, the mentor of Warren Buffet and one of the greatest investors of all time. Sound advice that applies to all.
Know your circle of competency. When you stray to far out from it you reduce your chances of being profitable. Being unguided in uncharted waters drastically increases your chance of mistakes and errors, while dramatically decreasing your likelihood of success.
At the same time constantly expand and explore your circle of competency because that's where your personal innovations will come from. It's where your most easily recognizable, scalable and profitable projects will be found. Never forget to constantly expand your circle of competency as it will give you an ever increasing area to mine in order to find the gold just beneath the surface.