Ha ha ha ! I read some answers and it gave me a good idea. What about becoming the President of my country ? Right... He's such a jerk, that would give us a break.
I would repeal the law about the retirement age (I was among the people that marched last Autumn). I would nationalize the banks (hey, give back the money the taxpayer gave you some time ago to save your sorry asses !). I would reform the Constitution to enable more participation from the people. I would have a real policy regarding unemployment (I am myself on the dole, but if I'm becoming the president, I would have a job then ! LOL). I would cancel the tax cuts for the very rich. I would... but you have a fair idea.
Oh, I would apologize to the Tunisian people for our government complicity with their own jerk of last president, which they were right to throw out. Our government behaved like a bunch of cowards but that's not surprising.
However... 2 problems arise :
1- in order to have an efficient Polyjuice Potion, you have to take something from the person you which to impersonate. That means you have to approach closely the person (in my example, the French president). That's the difficult part.
2- in my example, I would implement the exact contrary of his policy. That promised to be even more difficult.