Well, I think in any power-hungry job, whether or not it's a regular 9-5 to running a country, the only way to take it down is from the inside. I mean, if a couple of characters had picked up on it in the book, just regular wizards, it would've been overlooked. The only way to really make a difference is to have some power of your own - and if you don't have any, create it.

I think, as a whole, there's probably nothing that could've been done in the books unless they all banded together. The Ministry was looking out for each ohter when Fudge got so crazy and controlling because they were looking out for themselves. They could've lost their jobs, had their families put at risk, etc. None of them would help the regular man take Fudge down when they have so much to lose.

As for it being prevented in the future... I sadly think the only way to prevent this from happening is to try to trust the people you elect into office and keep them in check. =/