well Pierce, I'm glad you asked. this is one of those mundane things we learn one method for & then become complacent with despite the potential there are better & faster options we've ignored. let's explore all the most efficient selecting methods we have to ensure we use the best ones whenever appropriate.
making the most of shift
one cool feature of the shift key is that it will taper off the side of the selection closest to where you click. that is, if you are left of the center of the selection & click while holding shift, the selection will shorten from the left side. this becomes even more useful when you add in a held cmd key which will allow you to finely tune your selection without regard of the grid. this all makes more sense visually:
anchor & adjust is one great technique for making non-standard length selections. start by clicking the side of the sample that aligns with the grid then shift + cmd + drag to the point you want to select to. in this example, I didn't want any space between the two sounds so I select the start of one (conveniently lined up with the grid) & use the shift & command keys to select all the way up to the end of the previous sound. a simple delete time shortcut then removes the space in between.
there are many ways to make two clips the exact same length but here's an interesting one: click a clip's title to select it, then hold shift & press the down key. you can see we've selected a second clip without extending the selections length! in this instance, I use cmd + e to split the clip to the exact non-grid length of the first.
if you're rearranging your song, you'll need to make sure you're selecting everything including the master track - you don't want to leave tempo changes & master automation behind when you move your clips.
perhaps the quickest way to do this is to simply use the cmd + a shortcut to select EVERYTHING then taper it back using the aforementioned shift + click method to select only what you want.
an alternative, minimalist approach is to make one click at the start & top of your desired selection & make a subsequent click on the master track at the opposite end while holding shift. this of course yields a selection of all that falls between.
one of my favorite transparent features of live 9 was the easy method for selecting tracks you couldn't see. simply dragging your selection above (or below) the highest visible track will conveniently select everything above it.
here's an interesting one from u/fashionzoo: you can click the loop bracket to select all track within its range. this can be really useful if you're using loops to build up sections; you can easily select & arrange your selection using this trick.
moving & rearrangement
knowing how to make faster selections is great, but what's it all for? the most common reason for selecting full sections is to move them to make compositional changes. dragging a verse across your track seems cumbersome; it is often far more convenient to take advantage of time-editing tools.
ableton's cut time feature can seriously cut time out of your production workflow. one of the nice features of using time-editing shortcuts is that you don't have to worry about selecting all of your tracks - only the amount of time selected matters.
a similar & similarly useful technique is to duplicate time (using cmd + shift + d) in order to create a copy of your selection.
if you like learning new ways to use your keyboard, you can check out a full list of lesser-used, abundantly useful shortcuts here.
& that's it. I hope you learned something about making selections, Pierce. if you have a question or article idea, let me know on the twitter.