iOS: Double click Home Button
When you double click the physical 'Home' Button of your iPhone, the background 'running/idle' Apps will be brought up. Then, you long press any of those small icons; you will see little 'close' buttons. Most of time, you will surprisingly see a lot of 'running/idle' Apps here. Close them to free up some memory for the forefront App. There is another tip here. It supports multiple fingers touch, which means you can use 3-4 fingers to tap this area to speed up closing them.
iOS: Lock Orientation Change
Same as above, double click the physical 'Home' Button of your iPhone, scroll to the left until you see the following. Click the left most icon, which is the lock orientation button.
iOS: Quick Language Switch
Most of people just know click and change language. Change one by one click. When you press and hold, you will see the following. Hold and move your finger to the language you want and release. Doing so, you've got the language selected.
Mac: Resize Bar Icons
It is really difficult to find, if you've never been told. Look at the red highlighted spot. You know what to do. Try it!