I wish I'd known right off the bat how to code idiomatically in Python. You can pick up any language you like and start coding in it like it's C, Java, etc. but ideally you'll learn to code in "the spirit" of the language. Python is particularly relevant, as I think it has a definite style of its own.
While I found it a little later in my Python career than I would have liked, this excellent article wraps up many Python idioms and the little tricks that make it special. Several of the things people have mentioned in their answers so far are contained within:
Code Like a Pythonista: Idiomatic Python.
Enjoy!
