In a hyper-inflated economy, how do vendors know how much to raise their prices?
It's anecdote, but in this fantastic Planet Money piece on breaking Brazil's hyperinflation, there's a pretty good description of how in Brazil, shopkeepers would raise their prices multiple times a day. The strategies described (starting around the 10:00 mark, after the description of people running ahead of the guys with the price guns to buy things before they could raise the prices) include:


Assuming that inflation will run at some constant rate
Running around checking competitors' prices
Using the exchange rate against the US Dollar as a benchmark
Pooling information and colluding with competitors, agreeing to all raise prices by the same fixed amount each day.