Having good testers on your team is critical. Here's what makes a good tester:
Writes everything down: error messages, steps, etc. I mean EVERYTHING.
Good at informal testing: able to run with the system and find issues.
Attempts to isolate the problem. They go beyond just saying the test fails.
Good at defect tracking - systematically documents defects. Doesn't announce every time there's a defect.
The test lead pulls in people to review defects. Doesn't just declare a defect and hopes that someone will deal with it.
A tester needs to be humble and not gloat over problems.
If tester finds something majorly wrong, they don't make a big deal about it - no drama, no blame. Other people are paid to do that.
Good testers are hard to find!