Good software testers are a rare breed. There are also different sorts of testing and often a person can be good at one kind of testing but not at another. Here are some basic tips for choosing testers:
Don’t rely on a single person to do all the testing
Relying on a single person is putting all your eggs into one basket. You’re just asking for trouble. It’s very easy to get bored while testing and one person will, perhaps without realising it, miss important tests or features.
Don’t rely on the product developers to do all the testing
The product developers are simply too close to the product to see the wood for the trees. They will know the product back to front and will use the product features in the way they expect the features will be used. The product developers may not test the product in ways it will end up getting used by real customers.
Don’t rely on external developers to evaluate features
Using external people is good. They won’t be so familiar with the product. Using other developers though isn’t ideal. They are so computer literate they will, probably subconsciously, work around difficulties or things which might not be obvious to less savvy users.
Use women as well as men
Women often make very good testers. Women can be better than men at working in a detailed way. In my experience they are also less embarrassed in asking for help about something they don’t understand. This is a key skill for testers. The worst thing that can happen is a feature gets into the field, a problem occurs, and you find it was spotted in testing but not reported because the tester worked around the difficulty.