Make reference to Steven Johnson's "Farsighted", Daniel Kahnaman "Thinking Fast Slow", Chip/Stan Heath 'Decisive'
The Double Diamond example above is deceptive in that it appears realitively simple to follow. However, expanding your initial range of options could be the hardest part of your work. That's because human possess a strong tendency to anchor on one idea. Anchoring isn't unique to UX research, so there are many tools at our disposal that we can use to dislodge our work from this known gremlin.
Investigate your blind spots
There will always be some aspect of your project that you don't fully understand. Digging into your blind spots is first step in expanding your option set as it is the easiest to diagnose.
Remove ideal solutions
Challenge yourself to act as if the option you are anchoring on wasn't available. Anchoring on an idea is impossible if you remove it as an option.
Look for diversity in your data
Are you really talking to a diverse group of people or are they really similar to one another. Even if you are talking to your subjects individually, you can inadvertently fall into group think if you are not consulting people from different backgrounds and interests.
At the very minimum, read this post from Erika Hall. Small investments can pay big rewards if you take the time.