Unit Testing

One of the things that's I need to improve as a developer is my unit testing.  Now I try my best to not submit things to qa with problems and I never send things that have known problems but bugs invariably end up getting through; I'll never reduce this to zero but I think there is room for improvement (and probably there always will be).  I know there is a difference between diligent unit testing and spending too much time testing for developers, So I'm left looking for the happy medium.  What do other developers do in terms of unit testing?  My end goal is to have everything I've done in a NUnit style framework for unit testing, but this really seems to require a full adoption of test-driven development.  NUnit isn't available for everything that I do, a mix of: JavaScript, C/C++, XML/XSL, VB6, SQL/Oracle and .NET.  Is it best to write test harnesses for everything?  I'd like to, but the time needed to write these test harnesses seems immense when you get past the method level.  I'd like to start using code coverage tools, I think with some simpler tests and a good code coverage analysis I probably have a good idea of whether there are lots of hidden bugs im my code or not.  So here's my question, what do you find works best for you in terms of unit testing? While NUnit is great, I'd like to be able to do a similar process no matter what my chosen weapon is.  Granted this might not be possible in all cases.