Try broader terms
If your terms are very specific and you're not getting enough results, try including broader terms, for example:

Broader terms for "equine-assisted therapy": animal assisted therapy, physical therapy modalities

Try narrower terms
If your terms are broad and you're getting too many results, try searching on narrower terms, for example:

Narrower terms for "exercise": muscle stretching exercises, resistance training, running, swimming, warm-up exercise

Use OR for synonyms
Combine synonyms using the connector (aka Boolean operator) OR. For example:

physiotherapy or physical therapy

Use AND to combine
Combine different concepts using the connector (aka Boolean operator) AND. For example:

physical therapy AND hip replacement surgery

Combine OR and AND like this:

(physiotherapy OR physical therapy) AND hip replacement surgery

Quotation Marks (for most databases but not PubMed)
Use quotation marks to search an exact phrase:

(physiotherapy OR "physical therapy") AND "hip replacement surgery"

Truncation (for most databases but not PubMed)
Try using the truncation character (an asterisk: * ) to get more results. For example:

prevent* will find prevent, prevents, prevention, prevented.