Carry a small calculator
 

I found this most useful in the middle east and Indian sub-continent where bartering is the norm. You can swap proposed prices with the hotelier easily, without sharing a language.
Digits pressed onto a calculator have far less permanence than on paper. I think this gives a psychological advantage. Anyhow, its dead handy.

Or you could ensure that your mobile phone has a calculator function. everything must have at least two functions

go for the solar powered calculator, no battery hassels