Greenpeace and Amnesty aren't too bad though.
The charities that you really have to watch out for are the ones that outsource their recruitment and money collection to private companies. I remember there was a charity that I donated to regularly and the guy asked me for my regular donation and I asked him how much of my donation went to the cause. Turns out 50% was going to his company's fees! Taking into account the actual charty's admin fees, overheads, etc. and it's certain less than 50% of what I was donating was acually going towards the cause.