Be able to show you have done plenty of research to back up any claims or figures you plan to present. Don't pull figures out of the air. Don't assume you're going to sell enough to cover all your outgoings, do a best/worst case scenario - show what will happen if your business gets no clients or customers for six months (or a year for some industries). (UKBF is littered with stories of people who set up their businesses and expected to get lots of sales in month 1, only to find that didn't happen and, in month 2, they couldn't afford to keep going.)
Too many people go to investors trying to make out their business idea will be the best thing since sliced bread, but have nothing to back those claims up. Investors aren't stupid, they know what they're doing or they wouldn't have made enough money to be investing in businesses.
And point one on the outgoings, make sure you show how you are going to pay the investor back or how they are going to be rewarded from investing in you. I was told early on, so many people forget to show how they plan to pay money back.
What will really impress is if you can show you know your business idea inside out. You can answer intelligently, nearly any question that is thrown at you. But don't lie, don't try to wing it or answer with the first thought that comes to mind.