You'll usually get the lowest price before you get out of earshot because the seller knows she's losing you to the competition. If you don't hear a final price drop at the door, it means she can't go lower without losing money. That's her best price, and probably anyone else's with similar merchandise. You can negotiate better prices on leather coats or boots in July than December, and buying several items in the same place will always put you in a better bargaining position. Cruise the area first for comparative merchandise and prices, then buy as much as you can in one place on the way back.
Bargaining is always better when business has been bad.
The best bargains will be midweek, off-season, close to closing time, during the rainy season or when everyone else is on the beach. Remember, if you're the only customer a shopkeeper has seen all day, she won't want to let you get away. Buy something because you like it, and don't second-guess yourself later. Mexico itself is a bargain, and when you're happy with an item and its cost, you've gotten a good deal, no matter what you paid. If you're walking back over the border and don't know how to haul all the loot to your car, pay a buck and take one of the well-marked shuttle buses from Avenida Revolucion.