Most important for a programmer is to know how to test your code.... AND DO IT!

Any monkey can make code. Making code that works is the mark of a good coder.
This doesn't come with time or experience. Even the most experienced coder makes coding errors (and lots of them), but what distinguishes the good coders from the bad is that the good ones FIND their mistakes before releasing their code!

So, test continuously, test often, test everything, test every single module/function in isolation, test the modules/functions working together, then test everything again!
Make automatic test programs that you can run at a moments notice (or even better have a test machine checking out your code frequently and run them). Run them at least once a day (no, I'm not kidding). Remember, the sooner you catch an error, the less 'cost' it carries.
Use a code coverage tool to ensure you've tested all your code. Test methodically, not haphazardly 'poking' the code and calling that 'testing'.

I've been coding professionally for 15 years now. It still amazes me to see how many 'old' coders think they're coding gods just because they know every in and out of a dozen languages, and yet their code still fails frequently because they don't bother to test it (they don't need to, "because they know their shit" *roll*).

Learn to test your code well, and you're well on the way even if you don't know the language you're coding in as well as others.