Yes, I've signed all sorts of petitions, since they at least represent the percentage of people who'll actually bother to do so, but I wouldn't say that they have an effect in themselves. As Warbler suggests, it's more likely that change comes from an unrelenting process of petitioning, annoying your local MPs, writing to the Press, forming action groups, leafletting, fund-raising, gaining publicity, marching politely if noisily, protesting and waving banners and, if necessary, taking to the streets and being really unruly. I don't think just one feature of opposition will get things achieved, but unless people are willing to put their names or their actions to something, then absolutely nothing ever will.