Make sure you know the price of the food you are ordering. If you don't negotiate up front then it is culturally expected that you pay what the owner asks. The same goes for cyclo rides. The heaping teaspoons of white crystals going into your soup is msg. Before you panic, check out the latest studies on msg - apparently it isn't nearly as bad for your health as previously claimed.
Msg allergies are also much rarer than people think.
After several weeks in-country there is nothing in the world you want more than a big bowl of spaghetti with homemade sauce. Most questhouses will allow you to cook in their kitchen if you ask nicely and bring your own ingredients. It would be polite to allow them to sample the meal. It looks brown, smells like chocolate, and sometimes even comes wrapped in a piece of silver foil. Don't be fooled.