1. Search with keywords not with sentences. Google will try and guess what you are searching for, a database does not. Use keywords.
Good search: "medieval Europe" serfs
Bad search: What did the serfs do in medieval Europe?
2. Use quotations to make an important phrase stick together.
Example: "middle ages" or "medieval Europe"
3. Complete your search in 3-5 different ways. Use synonyms of your keywords to maximize your results.
Example: Search "middle ages", "medieval Europe", "the Dark Ages" in 3 separate searches. You will likely yield different results each time.