A few other notes that are worth, erm, noting. Since I know probably 95% of everyone here has never been to one, I'll give an explanation of someone the point as they would affect someone visiting a meeting who had never before.
Regarding the Sacrament, which the whole meeting actually revolves around, remember that you are not obligated or expected to actually "take" it, e.g. eat the bread and drink the water. If you would rather not and you are in between other people, just pass the tray to the next person. If you are a lone person on a particular row, a quick basic shake of the head or wave of the hand and the particular "passer" will understand and go to the next row. There is no stigma to doing this and you also sometimes see members doing this if they feel that are not 'worthy' that week. On the other side, if you are a non-member (hereafter "visitor"), and you choose to partake, you're not going to be damned or burned at the stake or anything.
During the normal course of the "ward business" section the meeting sometimes something called a "sustaining" or a "release" will happen. These are the formal methods to get the ward's approval on the appointment of a given member in the ward to a particular "calling". The sustaining calls for approval of said person to be put in said position, and a release is to extend appreciation to someone who is being released from a calling. During these, ward approval is garnered by the congregation raising their right arm in the manner of calling for votes. Though chance is given for any objections to be made, I have never been privy to an objection ever having been made. If you are not a member of that particular ward, you're really not supposed to give your approval (or disapproval), so unless you are a member of the ward at hand, you can just leave your arm at rest.
The first Sunday of the month is reserved for what is called "Fast and Testimony meeting, where the regular arrangement of speakers and hymns are replaced with members of the congregation giving public display of their spiritual feeling. This takes the form of a short (no time limit is enforced, but most of the time each person will take 2 to 5 minutes) public affirmation of faith called "Baring ones Testimony". All members of the congregation are invited at their own preference to share their feelings about the faith, the church, life and the like. These sometimes become emotional displays with occasional tears, which is why you'll always see a box of tissues next to the pulpit.
The sun rises, fish swim and Mormons sing. It's like the changing of the tides. If you show up, no matter how revolting you think your voice is, you are invited to join in. Don't worry, the amount of good singers is usually usually much higher than the bad so it actually sounds really good. But if you don't like to sing, don't want to or anything else, ya' don't have to.