People tend to participate or share their opinion under two circumstances:
1. It is something they feel strongly and negatively about (this is backed up by research and the average news article comment section); and/or
2. It is easy and convenient.
Even minor obstacles like having to manually type a website over clicking a link, or having to fill in some significant amount of information, or being asked questions they don't want to answer, will dissuade a lot of people from participating or providing their input on something.
Soliciting participation from the public needs to mean delivering turn-key opportunities for people to quickly provide feedback about the aspect of something they care about the most. There needs to be "an app for that."
Similarly, convenience needs to cross social divisions such as ethnicity, language, and other obstacles. Indigenous people may participate in our civil society and discourse in a way that is different than immigrants from Asian countries or from fourth generation citizens. Convenience therefore needs to also be adaptable and flexible depending on the target group.
Finally, it would be helpful to put information at peoples fingertips. Participation is most useful when it is informed. Giving participants the ability to quickly brush up on facts and studies about the subject in the same space they're providing their perspective might increase the quality and relevance of participation when it is solicited.