I've been using this for a little while now and it really takes no effort whatsoever. I simply use this instead of Google. You can make it your homepage or bookmark it.
Signing up gives the added bonus of being able to see how much you individually have raised, which is fun. But you don't have to sign up if you don't want to, and your funds raised will simply be added to the 'Anonymous' sum.
The amount you get for each search is quite small but it all adds up, especially when there are lots of us using it, and for not really doing anything, you can't really complain.

£558 pounds raised so far for the Psoriasis Association!