Try to support most of all local initiatives and when you buy souvenirs for example, always check that they are made in the country where you are.
Buy al smaller local businesess and avoid shops or restaurants which form part of international chains.
At local tourist sites, make use of local guides wherever possible.
Ask your tourleader about what customs there about bargaining.
With various indigenous populations you can bargain before you buy, while in other settings - Costa Rica for example -people are not used to this.
They simply ask the price as it is.
If you think some product is outrageously expensive, let your tourleader know or complain at official institutions (or on internet).
Part of sustainability is, that all parties involved receive a fair deal-including the tourist!