Pretty much what Violeta said. There's a super wide range of protocols -- you could probably find a domme willing to spit on you, if you're horny enough about spit to take that risk. Most Mistresses currently sessioning are pretty transparent about protocols, so you can do a little research, weigh your risks against your desires, and make a decision about who to contact.

In addition to the other protocols Violeta mentioned, some Mistresses are simply spacing out sessions a bunch -- if you see someone using that strategy, you'll definitely have to pay a deposit and you may be required to book a longer session.