SMILE. It’s like the monster ride at the amusement park. If you’re not having fun, get off.
Don’t bargain with the first person who approaches you (i.e., when looking for a taxi at the airport.) In general the further you go from the center of action, the cheaper the price.
Don’t be afraid to use walking away as a bargaining tool. You can always come back.
Don’t let pride get in the way of coming back (or Rule #3 is useless).
If you really want something and the price isn’t coming down, give in gracefully. (It’s that pride thing again.)
Don’t always assume you’re getting ripped off just because you’re in a third-world country. Sometimes the price they’re asking is the real one.
Bargaining is a time/money tradeoff. If you’re in a hurry, be prepared to pay for it.
Ask (Vietnamese) fellow shoppers the price of an item before you begin bargaining.
Always carry small bills. Otherwise all that work is likely to be for nothing.
Always bargain in native currency. Conventional wisdom says anyone who has dollars can afford to spend them.
If possible, keep going back to the same person until you’ve gotten to know him/her. Most third world countries have a name for this kind of relationship. The deal is that you offer your business on a regular basis and the seller responds by saving you their best tomatoes (fattest chickens, etc.).
No matter what your parents told you, when you’re bargaining it’s perfectly okay to lie.