I cannot ensure it, but I can try to create the right atmosphere for it.
It starts with me: I have to spread the vibe of “test positivity” and show what my work brings that is of value to the system (most important part of this system is not the software, but the people working to make the software).
The team: I try to create a good team atmosphere with regards to testing. I explain that I cannot do this alone because we will have better results if the whole team helps. I organise team testing sessions, pair testing sessions and praise people when they do good testing. I try to be open about how I test and I show what my exploratory testing uncovers.
Outside the team: This is usually the biggest challenge for me. I find that managers often have crazy expectation of what testing can do (assure quality, 100% complete, quick, easy, a phase, a checkmark on their Gantt chart, you name it). Getting stakeholders and other influencial figures outside the team to get what testing is about is tough. I try the same approach as with the team, but slightly different: shorter updates (I like the low-tech dashboard), being realistic and honest about risk, trying to explain the limitations of testing. I would appreciate insight from other people here about this area of testing.