Make sure that you press ALL the right buttons!
One of the things that he mentioned was the rather disappointing number of graduate economists who apply to the GES but fail the entrance test and interview process - around 50% of the just over 300 applicants that they get each year. Quite often this isn't because they are not sufficiently proficient in the technical skills that they need, but because they can't get the basics right. Of course the GES does want people who have technical skills in econometrics, but that in itself is not enough. Quantitative economics and econometrics is about more than just pressing buttons on a computer keyboard to generate unit root tests or run regressions. You have to be able to explain, interpret and communicate to people what you are doing. Although of course each year the GES employs about 160 excellent intelligent and energetic young economists, they could take more. But they find that too many applicants can't get the basics right. So don't neglect basic data skills, make sure that you are comfortable with index numbers, rates of growth and general spreadsheet skills. Make sure that you really understand basic economic concepts and principles like opportunity cost, the difference between real and nominal interest rates etc. A lot of what they do in the GES involves cost-benefit analysis. So if working in the GES sounds interesting and you apply there, make sure that you know about CBA and how it can be used in practical decision-making (especially how you would deal with conditions of risk and uncertainty).
To test your communication skills why not seek out a friend on a different degree course (maybe a medical student, a psychology student or a geographer) and see if you can explain in simple terms some basic economic ideas. Read one of the "poponomics" books like Freakonomics or The Undercover Economist (or maybe just browse through the blog that goes with the book).