When I am planning for a season, be it Lent, Advent & Christmas, Eastertide or Ordinary Time, I always start with the acclamations - both eucharistic and Gospel.   If we are to achieve the dialogue then the people need to be comfortable and familiar with the settings - if you chop and change from week to week then confusion is to be expected.   Sudden "surprises" have also had their impact; for example, in the days when First Holy Communion celebrations were planned without bringing me "into the loop" I was once given the list of music which included Chris. Walker's "Celtic Liturgy" - much used at the "big Church" and well-enough known at our little annexe for me to think no more about it.   At this point a should mention that I am a guitarist by choice and on this occasion there were no melody instruments available.   I launched into the the guitar introduction for the Sanctus and started singing the Celtic Liturgy.  About half of the assembly, however (the ones who hadn't got their books open), set off singing the Sanctus from Paul Inwood's "Gathering Mass" (Which we had been singing for the past several weeks)!  Suffice it to say that, after a brief period of interesting harmonies, we settled on the Walker but it was a good lesson for me to learn - avoid sudden changes "mid-season" if you can.

"Other parts, important in manifesting and stimulating the people's active participation, are also assigned to the whole congregation, especially the penitential rite, the profession of faith, the general intercessions, and the Lord's Prayer." (GIRM)
Now, I put this paragraph in because it puts the "Lord have mercy" into perspective.  In the days before Vatican II, the penitential rite was performed by the Priest alone as a personal preparation before he processed into the church.   It is not essential to sing this part of the Mass, although there are plenty of beautiful settings, and you might want to consider leaving it in spoken form for a period so that when it is used in a sung setting the impact and power is even greater.   What about using the spoken form in Ordinary before Lent so that a simple sung setting from Ash Wednesday onwards heightens our sense of this penitential season of preparation?  [Interestingly enough, I recently read that this is one part of the Mass which is being reconsidered in the light of the the new Roman Missal - the English translation will be with us in a couple of years, including a substantially expanded GIRM]  Similarly, you might only use musical responses to the prayers of the people on "special" occasions.   Not using music can often have as much impact as using it and these parts of the Mass should be considered carefully with respect to that.

"The Gloria is an ancient hymn in which the Church, assembled in the Spirit, praises and prays to the Father and the Lamb."
There are big settings and small settings of the Gloria - if you introduce a new one try to make sure that people get an opportunity to get a real feel for it before changing to another.   I tend to stick with one setting for a season (normally about 6-8 weeks).   It is probably wise to avoid using shortened arrangements such as the Peruvian or Taizé Glorias as these do not include the entire text as presented in the Missal.   I gather that the Bishop's Conference made a decision on the importance of retaining the text and meaning sometime in the last millennium but I wasn't paying attention so I missed it.  The Gloria is dropped during Advent and Lent - so please bear this in mind when you are planning!

"the eucharistic prayer ... is the centre and high point of the entire celebration. ...The meaning of the prayer is that the whole congregation joins Christ in acknowledging the works of God and in offering the sacrifice."
The Holy (Sanctus) and the Memorial Acclamation (Anamnesis) are our opportunity to share in the eucharistic prayer.   After the doxology we confirm it with a resounding "Amen" - if your Priest will sing the doxology then the people should sing the Amen.

"... during the breaking of the bread and the commingling the Agnus Dei is ordinarily sung... ... ...the invocation may be repeated as often as necessary to accompany the breaking of the bread and is brought to a close by the words grant us peace."

"The song during communion of the priest and people expresses the spiritual union of the communicants who join their voices in a single song, shows the joy of all, and makes the communion procession an act of brotherhood.  This song begins when the Priest receives communion and continues as long as convenient.  The communion song should be concluded in time if there is to be an additional hymn after communion." 
... "After communion, the priest and people may spend some time in silent prayer."
I tend to have a processional song and then space for silent prayer when everyone has received or allow space for silent prayer as we process to receive communion and then have a hymn.   I think that it is all too easy to fill every space in the Mass with sound and this is not to everyone's taste - certainly not my own.

There are some lovely settings of the Creed and, indeed, the Lord's Prayer but I would urge caution here as singing these can be exclusive at points in the celebration of the Mass when we are making declarations as one.   Perhaps sung settings of these two could be saved for other forms of liturgy, beyond the Mass.