As well as being legally open, data needs to be technically open, i.e. in a machine-readable format and as a complete dataset. Usually the easiest and cheapest way to do this is online.
- Open Data Handbook - The Open Data Handbook has produced a detailed guide about making data available, particularly focussing on online methods: http://opendatahandbook.org/en/how-to-open-up-data/make-data-available.html
- Making your data available on GitHub - This is a blog produced about making data available through GitHub: http://www.r-bloggers.com/data-on-github-the-easy-way-to-make-your-data-available/