I have been involved with more fandoms than I care to think about. After all those years of being on F/SF convention fandoms - including being on panels where such things were discussed, I decided to mentally create some canonicity rules to keep things straight. After many years, my point of view evolved into some very clear but rigid rules; most people are much more flexible, but I do have canonicity gradations to account for them. Moreover, the handling of contradictory information must be accounted for.
If you are familiar with Doctor Who or Star Trek, you know that there have been various releases of material in different media with debatable canonicity. For instance, the Star Trek cartoons were the most hotly debated point of canonicity in the Star Trek universe for years. Gene Roddenberry and Paramount went back and forth - usually to denigrate Doctor Who.
My definition of canonical is fairly simple: for a given franchise, if something is broadcast in an officially licenced medium with complete opening and closing titles at initial release such that a non-fan of the series can determine the series name and work out who had speaking parts and who were on the production staff, then it is canonical. Any re-edit must have been broadcast in the initial medium of release to be considered canonical. A remake via changing the special effects et al. is not fully canonical. Most importantly, the average or even casual fan of the franchise will typically be aware of its contents.
For a bit of context, in Star Trek, we had to determine which version of "Star Trek: The Motion Picture" and "Star Trek: The Wrath of Khan" would be definitive - the original movie versions, the ones that ABC broadcast with extra material and toning down of violence, the wide screen or not, etc. Similarly, there eventually were debates about the canonicity of the cartoons, the novels, etc. Doctor Who had the comics, the webisodes, the extended versions of "The Curse of Fenric" and "Silver Nemesis", the books, the audios, the specials for Children in Need, "A Fix With Sontarans, and so on. This makes these decisions easier: the original broadcast versions in the theaters for the movies and the original versions of the broadcast episodes are absolutely canonical.