1. Cross reference keyword searches in your library catalog with a search on Amazon or a similar site. When you look up a source on Amazon, it suggests works that people who were interested in your source were also interested in. I find this useful for two reasons - it returns potential sources without requiring you to identify the keywords most useful to your nascent search, and it offers a way of telling which potential sources are most valued. This is useful mostly for books, of course.
2. Search the call numbers surrounding a potential source. This will help identify thematically similar sources without having to rely on a keyword search.
3. Search article by article, issue by issue for the last ten years or so in the eight or ten journals that seem most likely to relate to your subject. This seems labor intensive, but in reality does not take that much time if you pursue it in a workmanlike fashion.
4. Skim your most recent sources first. Their bibliographies will both help you identify other, older sources and help you determine which older sources are most valued by your field.