How to bargain in Vietnam

You bargain to develop social relationships and to have fun. Saving yourself some money should be a secondary considerations.
Rule #1 SMILE. It's like the monster ride at the amusement park. If you're not having fun, get off.
Rule #2 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.
Rule #3 Don't be afraid to use walking away as a bargaining tool. You can always come back.
Rule #4 Don't let pride get in the way of coming back (or Rule #3 is useless).
Rule #5 If you really want something and the price isn't coming down, give in gracefully. (It's that pride thing again.)
Rule #6 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.
Rule #7 Bargaining is a time/money tradeoff. If you're in a hurry, be prepared to pay for it.
Rule #8 Ask (Vietnamese) fellow shoppers the price of an item before you begin bargaining.
Rule #9 Always carry small bills. Otherwise all that work is likely to be for nothing.
Rule #10 Always bargain in native currency. Conventional wisdom says anyone who has dollars can afford to spend them.
Rule #11 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.). 
Rule #12 No matter what your parents told you, when you're bargaining it's perfectly okay to lie.