true fasting...

Isaiah 58:6-9

⁶ "Is not this the kind of fasting I have chosen:
to loose the chains of injustice
and untie the cords of the yoke,
to set the oppressed free
and break every yoke?

⁷ Is it not to share your food with the hungry
and to provide the poor wanderer with shelter—
when you see the naked, to clothe him,
and not to turn away from your own flesh and blood?

⁸ Then your light will break forth like the dawn,
and your healing will quickly appear;
then your righteousness will go before you,
and the glory of the LORD will be your rear guard.

⁹ Then you will call, and the LORD will answer;
you will cry for help, and he will say: Here am I.

Fasting is not about afflicting our souls and neglecting our bodies in order to contort the will of God around ours. Fasting is not denying our needs for a season just to gorge ourselves later. Fasting is not an appearance righteousness nor a display of piety (Matt. 6:16).
Fasting is about contorting our will around God's. It's about denying ourselves so that we may bless others. It's the acts or righteousness and the display of humility; Christ-likeness.
So when I fast, my hunger or desire should remind me of the needs of others and guide me to satisfying them. When I fast I should not simply skip a meal, but provide it for another. When I deny myself sleep, I should actively provide rest and comfort for another; providing shelter or blankets for the homeless. When I skip a luxury I ought to find my way to a hospital or prison to comforting the afflicted.
When I fast it is about contorting my will around the will of God; to reorder my priorities according to His. To set myself aside, sacrifice my rights and serve fully for the sake of Christ and the edification of others. This is true fasting.