Hi Yub...yes its something which I find very erotic as well.

I haven't done this with my sub partner (yet)...but thanks for asking about the best materials to use.

A few ideas (hardly my own inventions) but know you were looking for some:

(1) the menu idea (which Oz mentioned)
scrawled across her stomach or back for example:

RENT-A -SLUT
Ad hoc Grope 25p
Blow Job 50p
Arse Thrash 75p
Tit Spank 1 pound
Cunt Whip 2 pounds
Tight Tit Cinch 2 pounds
Cunt Fist 3 pounds
Anal Rape 4 pounds
Bitch torture 2 pounds (per hour)

You'd probably need a sub about 7 foot tall to get all that on...but you
get the idea...and the low prices increase the humiliation.

(2) Action based instructions, which have also been mentioned
Slap Hard (one word per buttock)
Tug Til I scream (on her tits)
Ram it Up Here (over her pussy)
etc.

(3) Proprietary type labelling (general or specific body part)
Master's Tits
Property of Men
etc.

(4) General descriptive stuff written anywhere
Slut, Whore, Trash, Fuck Toy, Cock Rag, etc. (mmm loved Sperm Bank)

(5) Specific descriptive stuff
Public Fuck Hole (in front or behind)
Cock Park or Cock Sheath (one word per buttock)

There was a great story in the library...but I forgot both author and title, which had the sub obliged to call on 7 different doors in a hotel: with each visit, she got a different abuse, lost or had some clothing adjusted for her next visit, and received a body tatoo, and was obliged to walk the corridors of the hotel in whatever state she was left in, until she located the next door to knock on, each visit getting more and more extreme...it was quite a tour de force. Just wish I could find it again.