Find an organization that already exists and get involved in it. If one doesn't exist, create one.
For packaging, you'd have a few potential avenues: 1. Work to build support politically. Even small groups of 30-40 people (or smaller) are very attractive for local politicians to meet and speak with. 2. Reach out to companies directly. 3. Get earned media (possibly in conjunction with 1 or 2).
If you have small, achievable, reasonable goals, then it may even be a win-win scenario for all of the above, which makes it easier to make progress. For example, you could have a campaign goal of reducing x% of packaging to reduce y tons of waste.
It could be positive media attention for a company to say they've worked with a group like yours (or a small local company!) to reduce packaging and help save the environment (and it may even save the company money).
Finding ways for everyone to win and creating a big tent (as opposed to trying to influence by vilifying) tends to be more successful, in my experience.
There are a number of factors at play here:
Sometimes, I'm not aware of the issue
I don't know if the offending party is amenable at any level to changing their mind
I don't know if something is possibly against the law (e.g. I didn't know the AG had a say until I say this post)
I don't know if the authorities or avenues of dissent that I have access to have any jurisdiction over or effect on the offending party
I often have very little insight into how others feel about the issue.
I don't know how much work needs to be done to shift proceedings (e.g. how many people need to contact the AG for them to change their priorities)
I don't know at what level I need to focus my efforts on (e.g. community, corporate, federal, ngo, etc.)
Everything is scattered all around the place.
For example, I saw this good idea in another HN post: "Would it be so hard to use less packaging on foods, especially snacks?". I have an opinion on this, but I don't know how to effectively voice it.
Is there an existing system, or can we make a better system to most effectively turn our opinions on the matter into action?