Of course I'm a liturgy nerd. I care a great deal about what we pray and how we pray. Like many Episcopal Priests, I was "raised" to think that great precision is required when praying the Eucharistic Prayer. God help you (literally) if you say it wrong. The magic won't happen! Jesus will flee your feast!
So I find it impressive when people can extemporaneously pray the Eucharistic prayers and hit all the right points. There is an ancient precedent for this: the Mozarabic Rite. It requires a deep understanding, in my humble opinion, of the Eucharistic prayers to do it well, but it is certainly within the capacity of most priests who put their mind to it. The so-called Rite III liturgy in the American Prayerbook (1979 BCP 402ff) gives the presiding priest great discretion in how the Eucharistic Prayer is to be said, but still specifies the words for the Institution Narrative and a few other parts. Of course, the Rite III rubrics insist that it should not be used during the principal Sunday service of the community.
Certainly doing a Eucharistic Prayer off-the-cuff is stretching the bounds of what is "Anglican"--but that's perhaps what interests me about it. I see great potential for relational liturgy in the immediacy of such a prayer. It's the same principle as what happens when one learns to preach without notes or improvise of the organ.
I happen to enjoy broad episcopal permission when it comes the Contemplative Eucharists that I do on Wednesday morning and Saturday afternoon. Further, the Archbishop has told us (priests) that we "have the keys to the family car--just don't wreck it." So in that spirit I've been trying my hand at extemporaneous eucharistic prayers at my Contemplative Eucharists. The group that comes appreciates it. I find that it requires even more concentration and preparation than what I used before, which is even more than what it required to a regular BCP/BAS Mass!
To prepare for it, I think you need a couple of years of saying the Eucharist out of the prayerbooks. To this end, saying the daily masses at St. Mary Magdalene's was great training. It's like the 10,000 hours concept of Malcome Gladwell--you need to spend about 10,000 hours saying the Mass to master it. Maybe I haven't wracked up that much time, perhaps I've got about 3 or 4 thousand hours, though? You should have a few of rites basically memorized from repetition.
Then, you need to have a real command over the different parts of the Eucharistic prayers. You need to know what they are and why they are there.
Next, and perhaps most importantly, you need to be right with your personal sacramental prayer life. I mean that you need to be really confident in your own skin as a priest doing the thing priests do. So... that's my advice about that.
Now, here is an example of Rick Fabian (best known from St. Gregory of Nyssa fame) presiding at the Eucharist that ended our Music That Makes Community Conference in Atlanta a few months ago. Note that both the words and the music were made-up on the spot. We sing a Sanctus at the end in a paperless-style, but it wasn't our first time hearing/singing that particular song.
Now, there is a lot of craft in doing this well. For one thing, notice that Rick has a particular pattern in mind for how is going to chant the text. He is basically going up and a down a scale. Also, he has an idea in his head of the shape of the eucharistic prayers. He knows where is beginning from and where he is ending. He also knows some of the points he wants to hit along the way: thanksgiving for creation, rehearsing salvation history, the institution narrative, the epiclesis, etc.
BTW, I like the way Rick says "whores." It's a nice, sharp moment in the flow of the prayer.
I was never taught how to pray this way. I wasn't ready for it in seminary, anyway. These are advanced teachings. But it's definitely worth sharing because the spiritual results can be breath taking!