Dean Karlan provides charitable giving tips. Among them:

Organizations usually have transaction costs for processing each donation, and also spend time and money communicating with their donors – you might have noticed all those mailings piling up this time of year. So giving a large number of small donations is actually less effective than a few larger ones. Plus, if there is one that is doing the most good for the cause you care the most about, then every dollar you give to the one doing the 2nd best work is a dollar not given to the one doing the best work!

Earlier advice on charity here.