Brainstorm and jot down keywords for each part of your question. Try to think of every possible way an author/researcher might refer to your topics. Cross out words that aren't working and add to this list as you search.
Keep a search log or print your search histories. This will keep you from re-running the same searches and help you track how you've had to adjust your search.
This 4-minute tutorial explains how to use Boolean operators (AND, OR, and NOT) to build a search statement. (Click the button in the bottom right to expand the tutorial to full screen.)
Use an asterisk (*) after the root of a word to search for variant endings. For example, stretch* will find stretching, stretch, stretches, etc. This is called truncation.
Use quotation marks to keep words together as a phrase (e.g. "heart attack").
Look at the "help" menu in each database for more searching tips.
It is helpful to understand the difference between keywords and subject headings and the differences when searching with each.
I recommend that you try both types of searches. Start with keywords, notice the subject terms found in relevant results, and then use these subject terms to set up more focused searches. There is no one-way to search; it is an iterative process, whereby you try one search, make discoveries which will further direct the next search, and so-on. Some searches may be all keywords, some all subject terms, and some a combination of both. Here's an example of a search in PubMed, that uses a Medical Subject Heading and a keyword phrase:
Weight loss [MeSH] AND Overweight [MeSH] AND "resistant starch"
When you can't find any or enough information or when the studies you're finding are of lower-than-ideal quality, try some of these tricks for broadening your search:
When you are getting large numbers of results, try some of these tricks for narrowing your search: