This old post of mine, about international supply chain accountability, could be of interest: https://forum.effectivealtruism.org/posts/ME4zE34KBSYnt6hGp/new-cause-proposal-international-supply-chain-accountability

Hi Maxim, welcome to the EA Forum :-)

Before answering how, people in EA tend to first ask if. I.e. is the cause area of improving working conditions really a better use of our resources than something else (e.g. tackling poverty). To assess this, we could use the ITN framework. To illustrate this extremely quickly (i.e. without doing the analysis justice)

• Importance: Poverty appears to be a bigger issue than working conditions in the developing world.
• Tractability: At least some forms of tackling poverty are tractable, notably cash transfers. Improving working conditions does not appear tractable.
• Neglectedness: I don't know how much work is happening on tackling working conditions, but I would be surprised if it were neglected, given how much attention there has been on this topic. After all, it affects clothing and items that those of us in the rich world buy, which raises its profile much more than extreme poverty.

Indeed, some economists (e.g. Jeffrey Sachs) have suggested that we should want more sweatshops, as they are a valuable step in economic development.

I haven't looked carefully at the question of whether tackling working conditions in the developing world is high impact, but as the above illustrates, I would guess it probably isn't.

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However, what I've said thus far risks sounding unhelpful. You didn't ask for challenge on the premise of your question, you asked for how to go about making things better for workers. 

Again, I haven't thought about this carefully, but some thoughts off the top of my head:

• With this outcome (as with many others) it's easy to think of things that sound relevant, but likely hard to find ones where the intervention actually has evidence of working
• A full assessment should consider unintended consequences. For example, if you successfully improve working conditions for one group of people in the short term, other outcomes (either in the longer term or for other people) include:
• Firms may decide that if they are offering a higher wage, they should demand more qualifications, which may exclude the poorest people, for whom access to qualifications may be harder.
• Firms may decided that the purpose of locating their facilities in that country is no longer applicable, and may move (future) factories/offices elsewhere, potentially depriving the poorest people of job opportunities.

I guess if I were tasked with turning a substantial amount of philanthropic money into better outcomes for workers in the developing world, my first step would be to commission further research.

If I were really pressed to come up with an answer without researching it further, I guess I would suggest GiveDirectly. I don't think it directly solves the problem, and recipients of cash transfers may still end up working in sweatshops, but it's at least easier to decide whether you want to put up with it if you have savings.

I've thought about this a little. I think a business that helps eliminate the subset of managerial practices that are simultaneously bad for workers and the company is worth considering. 

For example, there seems to be an inverted U relationship between productivity (in many fields) and hours worked. For some businesses, reducing worker hours would both more them more productive and happier. Companies would likely pay to have external consultants identify such problems and rectify them. 

You could then reinvest the profits into scaling the business. 

Like any startup, I think this would have a 99% chance of failure, but I think it might still be worth giving it a shot because of how much good you could do if it worked (without a single philanthropic dollar invested)

There are workers' rights organizations in developing countries such as Bangladesh, like the Bangladesh Centre for Worker Solidarity. I don't know if it's possible to donate from a developed country though.

I also think about this, so thank you for raising the question! 

I personally donate to groups like APHEDA https://www.apheda.org.au/ on a hunch that they are effective.

My suspicion is that this community neglects promising opportunities in this space - and exploring it myself.

In aggregate workers' compensation is set by the marginal productivity of labour, and working conditions are a form of compensation, so you're probably looking for policies that will increase the productivity of workers. This could mean policies to encourage more investment or policies to increase the efficiency of existing capital (e.g. legal protections against expropriation, trade deals to help exports, reducing regulatory overhead, improvements to management practices, reducing crime, etc.)