1. If possible, don't go on the weekends. A cloudy or rainy Monday morning assures tourists will stay away and allows you to find the best deals
2. Don't make it obvious that you are a foreigner.
3. If you really like something, don't touch it. This shows the vendor that you're interested.
4. Have enough language knowledge to be able to bargain the price.
5. Walk away if the price isn't what you want. There's always a chance they could change their mind, and if they don't you can always return.