I wrote this as a comment elsewhere, and thought it perhaps worth reproducing:
You can get congregations to sing new music, but it takes a bit of work and cooperation from your organist/music director/whoever. I might add a few more points:
5) Try to make sure the first and last hymn or song of a service are tunes that people do know. I was taught that the first and last notes a musician plays will be what most people remember; this is also true of liturgy, and familiar, well-loved hymns at the beginning and end of a service will be less disorienting.
6) If the music is something people are going to be singing a lot (say, a hymn for Lent with different verses for each Sunday, or a congregational Mass setting), or if it's a bit difficult (syncopation, changes between triplets and duple quavers, awkward leaps in the melody, changes in harmonic rhythm and so on), it's worth offering a brief rehearsal at some point so people can go over the tricky bits. Try to make this short (ten minutes is plenty) and don't expect to get things perfect. Make sure it's at a time people can attend -- after a service is usually best.
7) Try not to introduce new music alongside changes to the general format of services, and don't introduce too many new things in quick succession, especially if it's a long time since the congregation has had to sing anything new at all.