This is really easy. It's the simplest and quickest way I know of to re-center yourself.
If you do this once an hour (set a beep on your phone to remind you), within several weeks it will become automatic and you'll do it without thinking.
- Let your spine be erect but not stiff. (If you're sitting, scoot your butt back in the seat.)
- Inhale up your spine
- Exhale down your spine
- Repeat for a few more breaths, if you like
Each time, notice the changes in how you feel inside. And notice the changes in how well you function in your daily life. Gradually, you'll build up a body awareness of what it feels like to be centered versus not centered. When that happens, your body will begin noticing and re-centering you automatically, without you needing to consciously think about it or make it happen.
Eventually, this will become second nature, just like maintaining your balance. Consider how easily you maintain your balance -- you've already practiced it so much that now you maintain your balance without even thinking about it, right? With practice, you will learn to stay centered in the same way.