Artur Malabarba over at Endless Parentheses has a handy optimization for commenting out a single line or, perhaps, lines at once. The usual procedure is to select the lines you want to comment and type 【Meta+;】 to call
comment-dwim. I use that all the time but what if you want to comment (or uncomment) just one or perhaps a few lines?
Malabarba's code comments/uncomments the current line or lines if a numerical prefix is given. He binds it to 【Ctrl+;】 which gives it a nice symmetry with 【Meta+;】.
In the comments, Scott Turner suggests a version that calls
comment-dwim if there is a region set and Malabarba's code otherwise1. You could then just rebind【Meta+;】to the new version and have the best of both worlds2. Either way, if you find yourself commenting and uncommenting lines a lot, you may find this bit of Elisp handy.