invest in long term skills; seek long term solutions to perennial problems.
Particularly seek skills which are flexible, which can lead to yet further useful skills, and which increase your general knowledge.
LaTeX. Save some useful macros or templates for reuse. Spend some time diagnosing any really troublesome LaTeX bugs, so that they don’t come back to waste more of your time in the future.
Be fluent in English (the international language of mathematics) in all four of its modes (reading, writing, speaking, listening). Basic proficiency in each of these is a good start, but you can always use more; this will help you communicate more clearly and more professionally.
Basic proficiency in other mathematical languages, such as French, is also useful, though nowadays with the advent of automatic translation programs, and with the rise of English in general, it is less of a necessity than it used to be.
Maple/Mathematica/Matlab/etc.; learn one
Operating OS. In particular learning how to download, upload, open, and organise files. Basic security measures; cleaning up after a virus or other security problem wastes a lot of your valuable time, even if you get an expert to help out.
Your email system! Learn about ways to quickly search your email archives, to automatically filter incoming emails into various categories, and ways to deal with spam and phishing.