Send a message to the government of the Islamic Republic of Iran via Amnesty International
At least eight people have now been killed by Basij militia-men in Iran for demonstrating against election rigging and the arrest of opponents of the government in Iran
Amnesty International UK have a page where you can send a message to the Iranian government asking them not to torture or kill Iranians or jail them without fair trial at http://www.amnesty.org.uk/actions_details.asp?ActionID=615
It includes a sample letter but it's better if you type in your own if you've got time. The best approach I could think of was to express shame that in the past my government helped overthrow the elected government of Mossadeq and backed the Shah, but to then say that the Islamic Republic's government cannot justify using the same methods the Shah used against their own people and that if they continue to they'll lose respect worldwide among Muslims and non-Muslims and go the way the Shah did. I've pasted what i wrote in below, but, again, better to use your own words.
"The Islamic Republic of Iran and its people are respected worldwide and I would like to say that I feel shame for my country's past involvement in overthrowing the democratically elected government of Mohammed Mossadeq and its government's support for the Shah's dictatorship.
However it cannot be right or justified for the Islamic Republic's government to treat it's own people the way the Shah did - to torture them, jail them without fair trial, allow Basij militias to murder them for exercising their democratic rights to freedom of speech and assembly.
I know that, as many Islamic scholars point out, democracy and free speech are not in conflict with Islam.
The actions being taken against unarmed and largely peaceful protesters bring shame upon the Islamic republic, as does the manipulation of election results. These are the methods of the Shah's dictatorship and if they continue to be used they can only result in Supreme Leader Khameini and his aides being overthrown by their own people the same way the Shah was, or in massacres which will shame the reputation of the Islamic Republic in the eyes of the Muslims and non-Muslims alike around the world.