To be both comprehensive and focused in your research, you should combine keyword and subject searching. If you begin with a subject search that works well, it can still be useful to expand your search with keyword strategies like the ones just described. And if you begin with a keyword search that finds some useful sources, it will be good to look for the subject terms found in the hits you retrieve and try subject searching also.
For example, a simple keyword search in our library catalog for self-help and medic* retrieves a few books, including the following one:
When you scroll down through the catalog description of the book, you will
find a number of subject terms, including Self-care,
Health which would seem a good subject way of searching for more books
on the topic. Searching the catalog by subject for Self-Care
Health will yield a listing of more than eighteen books.
In most article databases, similar techniques will allow you to switch between keyword and subject searches.