I compose a lot of documents in LaTeX. Homeworks, course notes, personal records, and meeting minutes all get typed up in TeX at some point. While the majority of the commands are quick to learn and easy to use, there are a few cases that popped up again and again that took a non-trivial amount of time either to type or to remind myself how it was done. Accordingly, some time around two years ago I started putting together a header file that I could include at the top of a document via \input{../physics_header.tex}, and it would take care of the majority of the packages and the like. While one can argue that I shouldn't have a bunch of packages included when I don't need them, the compile time is essentially negligible compared to the 3-second headache of having to go back and add "\includepackage{graphicx}" every time I want to add an image to a document.
After a bit I realized it was an ideal place to store my oft used, as well as my not-so-oft used, custom commands. I ended up bringing together the majority of the custom commands I'd written so far and organizing them by category. They mostly covered vector calculus, matrices, and quantum mechanics notation. I encourage you to take a look at the PDF I've put together regarding the commands, and if they're useful to you, grabbing a copy of my header and taking the useful bits for your own. Given that LaTeX allows me to include symlinked files, it's been very easy to sync the header across systems with Dropbox and continue using my commands everywhere.
That all being said, I have been cautioned about getting too comfortable with custom commands, as some academic journals that accept TeX files for compilation on their end aren't especially fond of them. It hasn't been an issue for me yet, but if it ever is I imagine writing a quick script to un-shorthand all of these things would be quick.