For Advent, I have typically used the Proulx Missa Emmanuel (and/or a homemade Latin version of the same). I think, being based on a tune with which everyone is familiar, it's easy to get away with bringing it in just for Advent, which is as you say very short.

For anything else, I think it would need to have some time to get traction. Say, bring back whatever you've done in Advent for Lent. But obviously in you're situation, you might not want to displace the Mass XVIII/Missal chants.

When I do this, I introduce one new "part" each week until everything I want to do from that setting has been introduced. Then I use the same setting for weeks on end. When I first introduced LeBlanc's St. Benedict Mass (which was the first one we did when the revised missal was implemented), we did it each week for probably 9 months. Right now we rotate between that one and the revised Heritage Mass. We are switching from Breaking Bread to Gather 3 and Celebrating the Eucharist later this year (it's official now; the books have been ordered) and I plan on making use of some of the better settings found therein to supplement what we've been using. Particularly interested in the revised Deutsche Mass and the People's/Danish settings from Gather, and the Austrian Mass from CTE. I think parts of Simplex are also included in there, and there is a lovely new setting in honor of St. Cecilia by David Hurd that's in the accompaniment binders that I'd like to consider if it's also in the missalettes.

I've found that infusing each new Mass setting requires a variety of approaches and strategies based upon the nature and properties of the Mass setting itself. Some, like the Proulx/Schubert and the Proulx Oecumenica have a sort of built-in intuition that congregations get rather quickly and allow the choir to sing readily in parts. Others, like Alstott's Heritage or a hidden old (unrevised) gem like Schiavone's Holy Family utilize some well-crafted motives that are reiterated with just the right amount of frequency. Schiavone's use of a primary motive instead of a repeated "refrain" in order to have a cohesive yet famliar through composed Glory was quite novel, satistfying both quire and congregation.
Newer settings like Nickel's St. Terese and JMO's Ostrowski have both intuitive elements in the soprano melody, but they're more subtley set and required much more time to be engrained, so using the men to sing the melody, later adding the women singing alto was a longer process before finally going to full voices. I've mentioned we're six month's into Chris Mueller's MR3, and so far this is the toughest sell. It's almost too gorgeous in its "compact unity" for a congregation to want to enter rather than listen to. And outside of the Kyrie and the Agnus Dei, I'm not sure that the Glory and Holy introductory motives have enough hook melodically to convince the unison congregation to enjoin. As our pastor recently (for the first time) asked us all to mark the seasons with varied settings, we'll move next to Clark''s Angels which I think will yield more melodic singing from the PiPs.
But I think an analysis of any setting would provide some insight as how to achieve initial and steady progress with the folks.

What about using a soon-to-be-introduced setting as an organ prelude for awhile beforehand?

For those who are introducing new settings before mass, or even hymns/propers, how exactly are you going about that? Do you send a cantor in front of the congregation at the ambo or other suitable place, and start annoucing? I am curious, as it feels very strange standing in front of the congregation before mass why everyone is praying.

When I introduce a new Mass setting I introduce it over a month. The first Sunday I start with either the Sanctus or the Agnus Dei, then add an element over the next three weekends. I also tend to continue the same setting used in Advent during the Christmas season. Since the Missa Simplex is not based on a familiar chant/hymn like Missa Emmanuel I would introduce it over the summer then go back to a standard for the remainder of Ordinary time and then bring it back for Advent/Christmas.

My parish went from Haugen's Mass of Creation to Proulx Missa Simplex in one week. It was a shock, but the choir learned it very easily in one rehearsal and the people are now singing it with no problems. I don't believe in changing the ordinary at all during the year. The ordinary is the one part of the mass that calls for "active participation" in all the Vatican II documents as I see it, and changing the ordinary only causes confusion. But that's my humble opinion.

Wonderful feedback, as always!
Well, we have been using the Heritage Mass since the implementation of MR3 with the exception of Lent... so that's going on two years. I did switch to form A of the Mystery of Faith from form B last Advent, and use form C for Lent.

I think it is good for congregations to develop a healthy repertoire of Ordinary settings because even the GR specifies which settings should be sung when. It also helps those that might be ignorant of the Liturgical Calendar (more than you think...) to understand something is different. But in response to MT56's post, I always keep the words of a colleague in mind: "When we (the musicians) are getting bored, the congregation is just starting to find confidence."

I do agree the the Missa Simplex is rather intuitive and would probably be one of the better choices for an Advent switch. Lots to think about....

What's wrong with, say, singing the Gloria right after mass, instead of a postlude or recessional? An announcement that such parts of the new setting of the Ordinary are being sung at the conclusion of mass will inform the congregation what is about to transpire ... and those interested enough to actually make the leap and join in singing the Ordinary will react favorably (if they like the new setting).

This depends on where you are: at my parish, people begin leaving their seats almost as soon as the final verse of the recessional starts (as soon as Father has processed out, usually). We are pretty much compelled to do any teaching before Mass because of this: nobody at our parish even stays long enough to hear a short postlude.

I always change settings at the Season. Yes, the difficulty is finding the right ones. But, if nothing else, you can certainly find 3 that are either known or easy to learn (Purple, Green, White).

Also- at my parish, and I'm sure this is the case in many other places- they already knew bits and pieces, but no unified Ordinaries. So, for example- they were very comfortable with the Sanctus from the Proulx/Schubert German Mass, but not the rest of the setting. So we've learned other pieces from it a bit at a time, and (for Christmas season this year) we'll do the whole thing (instead of a mish-mosh like in the past).

I find NOT changing Ordinaries at the seasons to be liturgically distressing- like if the priest wore green on Christmas, or if we had a sanctuary full of Poinsettias on the 17th Sunday of Ordinary Time.

As for the changing and selection of ordinaries itself, my last church was Episcopalian, rarely using a complete ordinary setting but rather selections by many different composers. I found a best practice of changing at the season (also at fall) and using only known settings, but including a single new setting. For example, the Gloria might be unfamiliar, but the rest would be. It was slow, but effective.

Gavin- same situation as mine. Same approach. Works.

Has anyone tried the idea of providing the "assembly" parts, or "congregation" parts on cards, so that those who want to can see the music, instead of learning it by rote?

I would never expect a congregation to learn Mass settings by rote. If you want them to sing it, print it in the program. Even if it's the Mass of Creation- print it.

For Ordinary Time, I strongly dislike long blocks of one setting of the Ordinary, unless it's to learn a new one. Otherwise, once it's in the repertoire, you should be able to rotate them (it also keeps the congregation more nimble to learn things - while I find MDs typically justify non-rotation to avoid burdening the congregation, more typically it's really to avoid burdening the choir rehearsal, and that's a formula for gradual slackening of standards for the Ordinary - if the musicians treat the Ordinary as musical wallpaper, then the congregation cannot be expected to do better).

changing the Ordinary to suit the season is hardly "musical wallpaper"

Regarding the Missa Emannuel, I've heard friends say that they don't really like it, and neither do I.

That does seem like what it's doing.

I can't get behind Missa Emmanuel. I want to. But I can't. First, I despise the fact that it repeats words. (I know it's allowed, and that my current Mass Setting does the same, but I still dislike it.) Second, it doesn't seem like the words fit the music. Third, the "antiphonal" structure is a nice idea, but one that I feel is executed poorly.

I quite like the Missa Simplex for Advent. I can't explain why, but when I first heard it, it immediately struck me as "Adventy." If I'm not mistaken, though, isn't Mass XVIII assigned to Advent, Lent, and the Requiem?

I've introduced XVII to normal, average congregations and it has turned out just fine. Just do it amazingly and people will love it.

Ok, last defense of (excuse for) Missa Emmanuel. I don't do it as written. I don't do the antiphonal repeats (usually), and I do it in my own Latin adaptation, so I can make sure the words fit [better]. I'm probably breaking lots of laws, and offending lots of good taste, but it's worked for me.

@Gavin - the practice cds, is it legally ok?

I do want to change mass settings for advent and recording the mass setting seems like a brilliant idea. But since our budget is so limited I was thinking of posting them online, non downloadable, in our Parish website (under music). So people can go and listen? Just wondering if its legally allowable?

I'm convinced that, contra the declarations of copyright holders, practice recordings pass at least two of the four tests for "Fair Use" (and the more important of the four, at that) and do not constitute a violation.

But that's just me.

I wrote a Mass setting based on Conditor Alme Siderum. It sounded Adventy to me, but I don't know if anyone else noticed.

Adventy... is that a new liturgical season?

Almost. It's the time from the day after Thanksgiving until late Christmas eve at every shopping mall in the country.

At a previous parish, I had one person who, on more than one occasion, complained of my minor-key preludes as being too "Halloweeny."

My wife now uses that term to refer to all things in minor.