MPs are obliged to respond to letters from their constituents so not only will it make them think about it, it will show them that this was a huge demo that reflected a very wide spectrum of public opinion (rather than just 70,000 anarchists!); if it is a Labour MP it probably won't do any harm to tell them that you want them out of government and will vote against them in the next election too.
Students have two MPs if they are away from home during term time and should write to both using the relevant address.
On a sidenote: it's good to see that Americans here realise that this was not an anti-american demo but was anti-Bush - it could just as easily be viewed as anti-British. It was the most amazing demo I've ever seen - extremely tolerant, peaceful and positive. I also hope it showed the Muslim world that the British people, and everyone we welcome into our country to live and demonstrate, are not against them either.