Ask Auntie Social: How do I run a competition on Facebook within their rules?

Facebook want to make it easy for businesses of all sizes to benefit but sharing, inviting or asking someone to post something on their personal timeline is a no go for a Facebook competition as set out by the Facebook Promotion Guidelines.
The rationale behind this decision is a technical one, as Facebook users can limit the visibility of the content they post to only friends or groups. Business pages won’t therefore have the ability to access all the entries that people post on their own timeline unless the entries are public.