Matthew 6:16-18 - Moreover when ye fast, be not, as the hypocrites, of a sad countenance: for they disfigure their faces, that they may appear unto men to fast. Verily I say unto you, They have their reward.
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Joel 2:12 - Therefore also now, saith the LORD, turn ye even to me with all your heart, and with fasting, and with weeping, and with mourning:
Matthew 6:18 - That thou appear not unto men to fast, but unto thy Father which is in secret: and thy Father, which seeth in secret, shall reward thee openly.
That last one is the best "tip", and my favorite.