Unit testing is essentially that first rung of the testing process, where individual units or components are tested to ensure they perform as designed. By running unit tests on your software, you give yourself a good foundation that will help when it comes to the remaining testing activities. However, unit tests of a poor quality will significantly reduce the benefit gained from running them, and can potentially introduce issues that could have been easily avoided.
As with most things relating to software, an efficient way to ensure quality in your tests is to follow best practice guidelines. So should you find yourself seeking quality, have a look at these testing guidelines, and you should find that by making a few simple changes to how you work you should begin to see an improvement.