Throughout the course of mapping I've slowly discovered incredibly useful hotkeys/strategies that I feel embarrassed for having taken so long to find. Many of these small things can be the difference between hours of work or the click of a button. I'm not sure this is the right spot to put this, but here's a list of a few useful tools/tricks that usually aren't covered in tutorials. I hope other people will add to the list (and refine my additions) as I'm sure I've barely scratched the surface hardly being better than a beginner myself. Some of them might be obvious, but they were things I missed starting out and had to learn over time.
Right click on selected objects and click flip horizontal/vertical to, well, flip horizontal/vertical
With a face selected in the face edit tool, alt+right click on a second face to apply the previous face's settings and align the second face perfectly with the first (especially useful for aligning textures on slanted/rotated brushes)
Keep everything to as large of grid units as possible, it makes things easier to deal with and improves vis compile times
Don't bother running vis for most compiles
Put rad in fastmode for non light-related compiles
Turn small detail brushes into func_details to lower vis compile times, do this early in production, not late or it all builds up
In tools>options>2d views set "default to 15 degree rotations" and hold down alt whenever you want to rotate by less than 15 degrees
Consistency is key; if you make a corner using one method, keep using that method, if you make a wall one width, keep using that width, etc. etc. (excluding of course variations for aesthetics) This is one I see needed A LOT when I look at beginner's maps, when you're mapping you should have OCD
Completely finish a detail's settings (visgroup, playerclipping etc.) BEFORE copying it all across your map (or use a func_instance)
Try to keep vertices on the grid even if it means you won't get a perfect cylinder/slant/whatever
If you have to clip/rotate something off the grid use a func_instance or group it with an on-the-grid skip brush (you can use the skip brush to move around the group without losing the proper off-grid alignment)
I hope these will be helpful for any beginners stumbling across this, they've been very helpful for me.