The best way I know to strengthen the soul is to do things that body and mind don't want to do.

Voluntary suffering.

This is especially useful for those who haven't even discovered their souls yet.

When things get really hard but we keep voluntarily going anyway, the different parts become easy to differentiate.

Body and mind will scream and whine and find excuses, but the voice of the soul will insist we keep going and fulfill our promise to ourselves.

Being able to fulfill the promise we have given to ourselves, no matter how hard it gets, is a great marker for soul development.