Take a look at subject headings for ideas to extend or narrow your search. Put phrases in quotes, and use truncation (an asterisk * in the Catalog) to search for alternative endings and spellings of words (e.g., iraq terror* will pick up terror, terrorism, terrorist, etc., while labo*r Britain politics will get results for both labor and labour).
Also, try narrowing your searches by combining key terms with a country name or region, like "civil war" sudan or "race relations" europe.
Princeton owns millions of items, but keep in mind that the library can also get you books from around the world. You can search the collections of Ivy League libraries through Borrow Direct or libraries throughout the world using WorldCat.org.
Searching think tanks and other NGO/civil society organization publications can be a very helpful complement to scholarly publications:
Harvard Think Tank search engine
Searching Articles+ is a great way to get directly to the full text of journal articles and ebooks we have access to. For even easier access (especially when you're off-campus), install the Library's new tool:
LibKey Nomad browser extension: 
It provides one-click access to journal articles from scholarly and publisher websites, Wikipedia, PubMed, and more.
