I strongly recommend Brian Skinner's Gumption Memo as a good read for those considering philanthropic decisions. Many of the ideas expressed below are derived therefrom.
I don't particularly care much about the question of how much to give–everyone has their own opinion on the "right amount," and that's a personal decision.
What I do care about, though, is how individuals go about deciding how to allocate their contributions. By this I mean the path we all have to travel from the things we value to the organizations to which we make out our checks.
I believe the goal of most people in giving to charity is to make the world a better place, for some definition of "better." Given one's own personal definition of "better," though, how best to distribute one's contribution to effect this change?
A simple analysis suggests that one should take a selection of efficient charities reflecting one's own values, and distribute one's contribution accordingly. Interestingly, this isn't really optimal. If one's goal is to bring the distribution of the world's philanthropic resource more closely in line with your desires, one really should identify the recipient that is most egregiously underfunded, and, presuming one's resources are insufficient to rectify this condition (as is true for the vast majority of us), just give everything to that one charity.
No one wants to do this, of course; it's frustrating to say "no" to organizations whose beliefs you share. Frankly, I don't do this either; I consider some fraction of my contributions to some organizations partially "selfish," in that they benefit me either culturally or geographically. There's nothing wrong with this, of course; most of the money we spend is essentially entirely selfish; a donation to a local arts group is still far more altruistic than the purchase of a new DVD player.
My definition of a "better" world is one where resources are consumed at a sustainable pace, all human beings have access to education and rich and diverse cultures, and human suffering is minimized. The "root problem" here is, in my analysis, overpopulation; as such the lion's share of my contributions go to IPPF/WHR.