I'm currently sming my school's fall musical (Into the Woods). We're a boarding school, so illnesses spread extremely quickly and sick kids leave campus until they are better, so we made every possible attempt to 'bus-proof' (or swine-flu-proof, in this case) the show.
One thing that I've found to be very helpful is to have plots at two different detail levels. For fly, they have a sheet with the songs that movement happens during and what happens, and a more detailed sheet with exactly when the movement happens and how fast. The first sheet is what they have up there during the show and are looking at, so they know when they can work on homework up there or something; the second could be used to write all the cues into a prompt book, if needed. Last night's rehearsal, the headsets stopped working part way through and both the people doing fly were home sick, but their replacements were able to use the more detailed sheet to get through the show.
For lights cues, I taught both my ASMs how to call a show, making them practise the hardest sequences. Hopefully, if something were to happen to me, they'd be able to call the show on their own.
One thing that we did realise we needed to communicate better were the prop plots. We had when the prop needed to be somewhere, but not how it was getting there from the previous scene, so when I had to send the ASM who normally supervises props to fix something else, we ended up not having Jack's harp for a scene it should've been it.