The "tourist tax" is quite difficult to circumvent. The best way is to make close local friends and get them to do all your ordering or negotiations for you. Normally food and drink prices are clearly written and are not over-priced unless you're at a tourist destination. Taxis are meant to always take meter so you should never take cabs unless they go by the meter. Some tourist-congested places are notorious for this (i.e. Platinum Shopping Center and Chatuchak Weekend Market), but it's quite easy to walk a little further from the main location and grab a meter cab. (At these places they also try to over charge locals, so it's not really just tourists :P)

Now, there will be instances where even local friends can't help. Such as entrance fees etc, if you look obviously foreign, you're going to be hit with a tax no matter what, but these places are usually tourist attractions and you're expecting to pay those anyways.