1. Use specific geographic location, theme, time frame, etc
2. Add 'gis data' and/or filetype: shapefile to the search string
3. Add' 'site:.gov' to look for reputable government GIS websites
US, State, County, and Local Government or Departmental Websites: Data.gov, Federal Geospatial Resources | US EPA, FCC Maps, National Geologic Map Database MapView, USGS National Map, U.S. Data by State - GIS Data Guide (by Purdue University)
US State Clearinghouse Websites: USGS Geospatial Data Sources, GIS Resources - Geospatial Data, Geographic Names Information System, USDA:NRCS:Geospatial Data Gateway
Non-profit, Academic, and other organizations: atmospheric science and oceanography - Repository Finder, Generalist Repository Comparison Chart - AGU