One of the main barriers to creating a culture of sharing in education is a lack of official approval. Here in Sweden teachers are often concerned about the perceived threats of sharing their materials; uncertainty about digital rights, fear of digital theft, lack of guidelines from above. Many teachers do share resources and make excellent use of the free material already available but the majority will not be convinced without clear approval from the top. It is impossible to make any significant progress in open education without leadership and a national strategy.
An impressive example of a national initiative to encourage sharing and collaboration is the Dutch Wikiwijs (English introduction, see the Dutch version). This is a site for sharing and finding educational resources and developing collaboration in Dutch education:
"Wikiwijs literally translates as Wikiwise. In a nutshell: Wikiwijs is an open, internet-based platform, where teachers can find, download, (further) develop and share educational resources. The whole project is based on open source software, open content and open standards."
Here you can create, share and adapt resources with guides and tips for new users. What is best about Wikiwijs is that it is aimed at all levels of education and not just for schools or university. The whole site has a Creative Commons attribution license giving maximum freedom for sharing. Here's the introduction video.
A similar initiative, though limited to schools, is the Norwegian NDLA (National Digital Learning Initiative) that has become highly successful and is used by most teachers in the country.
Once you have established an arena for sharing that is clearly sanctioned by the responsible authorities teachers will be much more likely to use them. Credibility and quality are vital factors to encourage teachers to adopt OER. Without them it will continue to be a fringe movement for enthusiasts only.