Subject Terms can help you identify effective keywords. Most databases list Subjects in their search results.
For example, in OneSearch:
1. Do an initial search.

2. Under Refine Search click on Subject.

Using ANDNote: in most, but not all, databases, the AND is implied. For example, Google automatically puts an AND in between search terms.
Example: renewable energy AND China

Example: renewable energy OR solar OR wind

This operator is great when you notice your search term is used in different disciplines. You might use this example when researching peacekeeping in the context of interpersonal conflict but your search results in documents about international conflict.
Example: peacekeeping NOT United Nations
Adapted from SAIS Library, Johns Hopkins Univ. "Database Search Tips" Guide (no longer extant).
Use quotation marks or parentheses around search words to search for a phrase. Otherwise most databases will find records that include both terms, but not necessary the terms as a phrase.
Examples:
"middle east"
"united nations peacekeeping forces"
"civil society"
Adapted from SAIS Library, Johns Hopkins Univ. "Database Search Tips" Guide