The problem many people, particularly students, face is that we want to help or support charities but let's face it, it's rare we have much money or time left over each week. I don't believe this is selfish, it's just the way humans are. From the time of the caveman we have been conditioned to preserve and store our food / money / tools in case of drought, famine or flood, it's a defense mechanism we've used to survive through centuries.
So I know how it feels when you're walking down the street and you see the smiling faces of yet another bunch of charity collectors making a beeline for you. Quick avert eye contact, pretend to answer your phone or run across the other side of the street (yes I have done all these). Personally the reason I find this form of giving so intrusive is, basically, I'm on the middle of the street, I'm going somewhere, I don't know you. Do I want to give you my credit card details, no. Do I want to sign up for an indefinite amount of giving each month, no. I'm not sure if i can pay my rent next week!
Instead I read the above mentioned book and it provided me with a great number of resources for giving what I can, when I can. Another great thing about many of these charities is you can choose the cause or person you would like to give to; I most prefer to give to educational causes for girls, fistula medical treatment in rural areas or micro-lending in Afghanistan which helps numerous people build themselves up financially and socially.
These are some of the best I've come across:
CARE a leading humanitarian organization fighting global poverty, places special focus on working alongside poor women.
Global Giving connects people who have community and world-changing ideas with people who can support them.
Kiva's mission is to connect people through lending to alleviate poverty.
Another issue is that nowadays, it is perfectly natural to be skeptical of how much of our money is going to people who are in need and not just to administration and overhead costs. Many companies (and some of the most well known) are notorious for providing less than 20c per dollar donated toward people who actually need support.
There is now a great website called Charity Navigator which is an independent financial evaluator of over 5000 charities. The basic idea of this website is that they track where donated money is spent within charities and provide detailed information to would-be donator's so they can make informed decisions on where they would prefer to help out. It's a great way to know your money is being used in the best way possible.