“When you look at the crucifix, you understand how much Jesus loved you then. When you look at the Sacred Host, you understand how much Jesus loves you now." 
— Mother Teresa

“When you are attending Mass and other religious services, be very reverent when you stand up, kneel, and sit. Perform each action with great devotion. Be modest in your gaze, and do not turn your head this way and that to see who is coming or going. Out of reverence for that holy place, do not laugh or look around to see who is nearby. Try not to talk to anyone unless charity or a strict need requires it.

If you are reciting a group prayer, pronounce the words of the prayer distinctly, stop for the pauses, and never hurry through the prayer.

In short, behave in such a way that all the bystanders are edified and, because of you, are moved to glorify and love the heavenly Father.

When you leave the church, have a calm and collected demeanor.”

-St Pio in a letter to Anitta Rodote, July 25, 1915

Excerpt from Padre Pio’s Spiritual Direction for Every Day pg 192

How to behave during Mass, by Saint Padre Pio

Enter the church in silence and with great respect, considering yourself unworthy to appear before the Lord’s Majesty.  Amongst other pious considerations, remember that our soul is the temple of God and, as such, we must keep it pure and spotless before God and his angels. Let us blush for having given access to the devil and his snares many times (with his enticements to the world, his pomp, his calling to the flesh) by not being able to keep our hearts pure and our bodies chaste; for having allowed our enemies to insinuate themselves into our hearts, thus desecrating the temple of God which we became through holy Baptism.

     Then take holy water and make the sign of the cross carefully and slowly

the full letter: