Mini bottle of mouthwash for a quick rinse after a BJ if I nip to the bathroom to freshen up. Nothing like stale cock breath to make me want to kill myself halfway through a three hour booking. Blagh.
Also, condoms in different sizes if you haven't met the client before is something I learned after desperately trying to squeeze a massive, erm, appendage into a normal sized condom once..... in fact, I just got back from a tour in the US and only used large sized ones, he he he! Well, I am laughing now, but trust me, I hobbled to the airport.