Most of the time, people are surprisingly cool about it if you're upfront! If they're not home, I'll leave a note along with the broken item. If its something like glass, I always do an extra detailed job in that room to assure them that there aren't any shards. If it was something fancy-looking, I take a picture with my phone so I have a record of what I broke. Make sure the customer feels as though you will report any accidents to them, that way, when their teenager/hubby/cat breaks something, they are less likely to blame you.