Why bother with subject headings?
It’s true that you can find sources on a topic by doing keyword searches. But if you limit yourself solely to keyword searching, you are likely to miss important material on your topic. If you only need two or three books, you can probably find what you need by doing keyword searches, but if you are doing historical or more indepth research, you can’t afford to miss critical material on your topic. For a comprehensive search on your topic, search with subject headings as well as keywords.
A good way to identify subject headings for a topic is to do a keyword search in the online catalog using terms you think describe the topic and try to identify a few relevant books. Look at the full record for those books to see what subject headings were used, then do another search on those headings.
As a rule of thumb, use fairly broad headings, as well as the specific ones that describe your topic, in order to make sure you haven't inadvertently eliminated relevant material that is contained within works of larger scope. Most likely you will find multiple headings to describe your topic, and you should use all of them. You can narrow your search in the online catalog by combining subject headings (as a phrase) with keywords, using the “Advanced Search” option.
Some Examples of Subject Headings
FOR RELIGION
United States--Church history
United States --Church history --20th century
Protestant churches --United States --History
Liberalism (Religion) --United States
Liberalism (Religion) --Protestant churches
Fundamentalism --United States
Evangelicalism --United States --History
Evangelicalism --United States --History --20th century
Evangelists --United States--Biography
Revivals --History.
Revivals --United States
African Americans --Religion
Pentacostals
Pentecostal churches --United States
Pentecostalism --United States
Millennialism
Religion and culture --United States
United States --Religious life and customs
Religion in the public schools --United States
Universities and colleges --United States --Religion
Christian education --United States
Radio in religion
Television in religion
Popular culture --Religious aspects --Christianity
Religious broadcasting
Christianity and politics --United States
FOR JUNKYARDS AND SALVAGE
Ragpickers -- United States
Salvage (Waste, etc.) -- United States
Recycling (Waste, etc.) -- United States
Waste products and building materials
Construction and demolition debris -- Recycling
Building materials -- Recycling
Poverty -- United States
Equality -- United States
Unskilled labor -- Supply and Demand -- United States
Labor Market -- United States
Unemployment -- United States
Temporary Employment -- United States
.
Encyclopedia of the American Religious Experience by 
Archaeology and Bible History by 
Going Green: True Tales from Gleaners, Scavengers and Dumpster Divers. by 
Minimalist Homeschooling by 
Survival Theory: How to Survive the End of the World on a Budget by 
Survive ANYTHING: the Ultimate Prepping and Survival Guide to Perfect Your Survival Skills and Survive ANY Disaster, ANYWHERE in the World! by 
When Homeschooling Gets Tough by