1) Opening/Peeking Doors:
CBA has a function in its controls called "Open Door." You can set this button as you want, walk up to a door, press it, and the door will open or close. If you hold this button and use your scrollwheel, you can slowly open it which allows for bouncing grenades off the door into the room.
2) Easy Vector-DAGR Use:
The Vector 21 and Vector 21 Nite can be paired with a DAGR to use the DAGR's GPS in conjunction with the Vector's laser range/azimuth finder to give accurate 10-digit grids and altitudes for targets. However, many people have trouble hitting both the TAB and R keys at the right time to make this system work.
In ACE Equipment controls, these are cited as two different keys: Vector Azimuth and Vector Range. If you set both of these to one key (TAB in my personal case) then you can JUST hold that one button down and it will do both functions.
3) Graphics Memory Flush:
A hidden gem hailing from at LEAST Arma 2, the in-game function to flush your video memory. The steps are as follows:
Hold LEFT SHIFT and hit the NUMPAD ( - ) key.
Hit "F" "L" "U" "S" "H" on your keyboard (NOT typing them into chat).
Your screen should flash to a loading screen while it flushes.
4) Faster IR Pointer/Flashlight use:
The rifle-mounted IR Pointers (PEQ boxes) and Flashlights are amazing tools to use as an infantryman at night. However, leaving them on for extended periods can be distracting and detract from the ease-of-picture of the battlefield. As such the best common practice is to use the IR Pointer only when ready to engage a target to avoid the "Star Wars Lightshow" effect on a battlefield. However, reaching up to press K on the keyboard can be time consuming-- especially in a pinch.
If you have a multi-button mouse, assigning the Lights On key to one of those extraneous mouse buttons can save time and create an easier way to flick that laser or flashlight on or off.