First, a 'flash demo' is in the works for 1pm today outside Parliament (see 11.10 update by The Guardian). Details are still emerging, and it may run out of steam before it starts. Note that this would be an unofficial protest and would therefore attract police attention.
Another protest will take place outside News International's offices in Wapping this Friday at 1pm. More details of 'Rage Against the Murdoch' can be found on this Facebook page or by following @cantstopanidea.
The most effective protest, however, might be to review your subscriptions to News International products.