If you weren't aware of the slide collapsing feature, you might find it helpful, as you could collapse all the slides of one song presentation and make moving the slides much easier. To do this, go into the Slide Navigator view (where all the slides appear down the left side of the window. Select all the slides except for the first one and drag them over to the right until a blue line with a downward triangle appears under the first slide. Release, and all the slides will be collapsed under the first slide (there will be a disclosure triangle beside the first slide that you can click to hide all the other slides).
You can drag and drop this single collapsed slide to a new presentation, and it put all the slides there. Thus, one possibility is instead of having multiple presentations, each for a single song, put all the songs, in collapsed form, into a single "master list" presentation. Then, to build a new weekly presentation, just drag and drop the single collapsed slide that contains all the slides for a given song. In this way, you will only have to move slides between two presentations, the "master" that has all the songs, and the new weekly version.
Alternatively, you might consider whether Keynote is the best solution for you. One long-time contributor here, Brian Peat, has compiled an
extensive list of software packages that are designed specifically for presenting song lyrics for worship services. You might find that these have features you could use.
PowerMac G5 Mac OS X (10.4.4)