Make sure you keep sick bags in your car. You can buy them at the discount chemists for a few bucks for a pack.
And don't rely on pax to tell you they are feeling unwell. If I have a pax who's drunk and I think there's any chance of them being sick, I diplomatically tell them to please let me know if they don't feel well as I have sick bags. If they're travelling with a friend I usually pass a sick bag to the friend and ask them to keep watch so they don't get stung by a $200 cleaning fee.