I'm no expert when it comes to etiquette at Mass, but I, at least, can recognize a problem when I see it. Here is my top fifteen list of things we really shouldn't be doing (or sometimes should be doing) while we're attending Mass.
1. Genuflect towards the tabernacle before taking your seat. This is a display of reverence, which acknowledges the real and mysterious presence of our Lord Jesus Christ.
2. Avoid talking loudly in the sanctuary. Unless we've actually been asked to speak to the congregation, let's keep our voices down to a reverent level. For those who are trying to worship, it's a distraction within Holy Mass. There's a social dimension to Mass, but that shouldn't be our main reason for being there.
3. Dress appropriately. Mini-skirts, for instance, don't belong in the sanctuary. They say "Look at me!" when we should be saying "Look at Him!"
4. Unless unusual circumstances exist, come to Mass in clean clothes.
5. Ringing cell phones and pagers distract, too. Turn them off.
6. Don't eat in the sanctuary. This should go without saying, but, besides being poor form, it breaks the fast.
7. An attitude of reverence and respect is necessary when coming forward in the Communion line. Don't expect the priest or Extraordinary Minister of the Holy Communion to offer the Eucharist otherwise. (When I was serving once as an EMHC, I had a teenager casually extend his hands which were entirely withdrawn inside the sleeves of his sweatshirt--and he didn't even make initial eye contact.)
8. Don't take the Eucharist in your hands to dip yourself into the chalice. This is called intinction, and, according to the General Instruction of the Roman Missal, the act of intinction is reserved for the priest, then placed on the tongue of the communicant. (287)
9. Keep your kiddos under control. (A good book on this topic would be this one from Michael Rayes.) We appreciate you bringing your kids to Mass, and we know it can be hard sometimes. Most of us are very patient, but when the priest can no longer be heard, it's a good indicator that it's time to take your child out for a bit.
10. It's inappropriate to leave Mass before the priest. In fact, why don't you stay and sing the dismissal hymn? Are we keeping you from something...?
11. Don't be in such a rush to leave that you force others to exit the pew earlier than they would prefer. That's pure and simple selfishness. (Had an older lady do this to us once. This was the same lady who put the kneeler down on my cast when my leg was broken.)
12. As far as the music director is concerned, please don't ask the congregation to sing a hymn which fails to convey the message it purports to offer. Understand that words mean things, and that many congregants will stop singing if the words fail to measure up.
13. As a worship leader or priest, try to avoid jargon of the day. Environmental stewardship is one thing. Teaching on recycling methods at length is probably a step or two too far. While we should be engaging the present culture, our eyes at Mass should be turned more to the eternal than the temporal.
14. As you pull into or out of the parking lot, remember where you are / have been. Let your life be a reflection of Christ outside the walls of the church, too. This means, for instance, helping a person you see collapse in the parking lot or avoiding laying on your horn as you grab the desired parking spot.
15. For professing Catholics, confession is not a "recommended" activity, but a required Sacrament. Remember also the warning of 1 Corinthians 11:27 concerning the taking of the Sacrament in an unworthy manner. As Bishop Fulton Sheen warned in one of his latter talks, this is a serious offense and places both your spiritual and physical health in real danger. Speak to your priest for more information.