So, sign the petition, write to your MP to urge them to support the campaign and tell everyone you know to get involved too. If you have a blog, then blog it; if you're on Twitter, tweet it ... you get the idea.
For further encouragement, here's a message from Simon Singh and Sense About Science:
"You know we joined up with free speech organisations to form the Libel Reform Coalition in December and that we have been working hard with many of you to highlight the impact of England's libel laws from my own and Peter Wilmshurst's cases to the chilling effect on medical journals and science blogs and other writing. This has begun to focus political and legal minds on the problem but we now have very little time to go before election manifestos are published. We need a commitment in those manifestos. Many of the politicians we've met don't dispute that the libel laws are unfair and against the public interest, but they won't commit to changing them."