The IRS provides a number of resources on tax deductions at http://www.irs.gov/charities/contributors/index.html
IRS Pub 526 ("Charitable Contributions") provides 20 detailed pages on the issue:
http://www.irs.gov/pub/irs-pdf/p526.pdf
For information on individual states, check the webpage for the revenue department in the relevant state.
For statistics on charitable deductions by level of household income, see:
http://www.irs.treas.gov/taxstats/indtaxstats/article/0,,id=96981,00.html
For statistics on charitable deductions by state, see Profiles of Individual Charitable Contributions by State (various years), available on our website under Statistics.
KEYWORDS:ITEMIZERS, CONTRIBUTIONS BY INCOME