In a hyper-inflated economy, how do vendors know how much to raise their prices? You regularly see stories about hyper-inflated economies. The common feature is that as prices spiral out of control, stores continually raise their prices to match the level of inflation, leading to insane costs for basic items and with note-values being increased to match prices. 

Let's say that I'm running a cake stall. How do I know that I need to be charging 100 trillion fakedollars per cake rather than 200 trillion? Where do normal vendors get that kind of in-depth market info?
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.