Searching for information at postgraduate level has to be precise and thorough.
Successful literature reviews are created from searches using well chosen keywords and phrases over a broad range of resources.
To conduct a precise and thorough search you may need to use a variety of techniques, from Boolean logic to truncation and proximity searching. Many of these techniques are explained in this printable document.
Not every technique is supported by every resource, and the ways in which the techniques are used can differ, so always check the help available for each individual resource.
If your search is continued over a long time you might find it helpful to keep a diary or journal. This can help trace changes in the direction of your research, and record useful search strategies, keywords and phrases.