He said that the first thing he learned in business was, when someone asked if he could do something to always answer yes regardless. He said afterwards you can say to yourself "f*@# how on earth am I going to do that!?" By saying yes you've set yourself a challenge - you don't want to let down your customer so you always work out a way to achieve it. The flow on effect from one "Yes" can be HUGE - as can the negative flow on from one "No" and you never miss out on an opportunity by taking that route.
So don't let fear and uncertainty hold you back and be the motivation for saying no. If you do say no make sure its for the right reasons.