Your mind is who you are; you can improve mental focus with a few simple practices while standing.
When you stand in the second position and find that you can stand for about 7-8 minutes you may find that your mind may begin to wander and that there may be some pain in your joints especially legs and arms when you get to this point it is time to bring the power of your mind into play.
The simplest way is when you find that a part of your body your arm or your leg for example starts to hurt just simply tell it to relax say it in your mind and you will soon find that your body reacts it is as simple as that. Try it and see.
Another simple thing that you can try is imagination when you are standing and find that your legs feel like giving way give your self a mental seat to sit on this can be anything that you would imagine to be soft and comfortable i.e a soft chair a cushion or a balloon. If you arms begin to tire imagine that there are soft balls of cotton wool or tennis balls beneath your arm pits and simply imagine that you are resting stopping them from falling down.
Below are a few exponents of standing like a tree.
This video shows that using the mind to and movement can be used in any style of martial art. This is Japanese