My main piece of advice would be: don't be afraid to rewrite your own code. Look at stuff you wrote even a month ago and you will see flaws and want to rewrite stuff.
Make sure that you understand some fundamentals: collections, equality, hashcodes etc. These are useful across pretty much all modern languages.
Depending on the language you use - use lint and metric tools and run them over your code. Not all their suggestions will be applicable but learning which are important and which are not is important. E.g FindBugs, PMD etc for Java.
Above all refine and keep refining your work. Don't treat your work as abandonware!
