Mine is to take a ruler and read every document you work on line by line before you send it to a senior associate or partner for review — and make sure you check every cross reference in every line when you come across it. It sounds totally unnecessary (you’re a law school grad, after all, and that’s fifth grade stuff, amirite?) but it’s a foolproof way to catch every little typo and definition and inconsistency so more senior lawyers don’t have to fix them for you later.
Oh! And also, if you're a Luddite like me, in between your bar trip and first day of work, take an online tutorial or read one of those "how-to" books on Excel/PowerPoint/Word. Again, super basic, but you really don't want to have to ask some junior banker to teach you how to use the Excel spreadsheet they've just emailed you...