Use a Variety of Tools ~ Using more than one tool insures better results. Even if you only need a couple of articles - you may find a better one by trying multiple tools. Look at the characteristics of each tool and choose a variety
What is its Knowledge Base? What Universe is it searching?
What are its Search Features?
Natural Language Processing
Keyword Searching
Field Searching
Subject Headings Limiters
What is its Authority?
Is it limited to Peer Reviewed Journals or Scholarly Sources
Are the results verified before being added to the tool? Or do you need to verify?
Conduct a Variety of Searches in each Tool- Try different keywords, field searches, and limiters. This gives you the best chance of finding good articles.
Be Iterative in your Searching - This means don't just search once, but come back and search again after you've read some things, after you've started writing, after you've collected data. When you come back later, you have more ideas for searching and different things will pop out at you.