I wanted my canonicity rules to be very rigid and limited for several reasons. One is that I hate canonicity debates - rigid, minimalistic rules for fully canon material will take care of all situations aside from flat out contradictions in continuity. Another is that it allows for more fans of a franchise to participate - just because they aren't as knowledgeable as the Big Name Fans does not mean that their opinions may not be better than the BNFs. Before we discuss how to handle continuity gaffes, let us see these canonicity rules in action.
What about the stuff about the kid that dies in ST:TWOK - is he Scotty's nephew? This is "essentially canonical" - it is commonly accepted but, very strictly speaking, it is not. In Doctor Who, I put the extra material for "The Curse of Fenric" and "Silver Nemesis" in this vein. Many average or even casual fans of the era will know the material.
How about the Star Trek cartoons? I figure the 6 of 7 of the mains were there, Gene Roddenberry got paid, and most of the writers were from the series, so I always considered them canonical, but I got debates over that because Filmation made them. But fans of the era were well aware of them.
A few things now are cut and dried. For iCarly, the extended versions of "iSaved Your Life" and "iParty With Victorious" are fully canonical, but alternate versions because they were broadcast in the same medium as the originals. You can expect average fans to be well-aware of their existence and knowledge of the material; casual fans will have some knowledge of their existence, but probably not much of the material.
Now it gets trickier. Are iCarly's webisodes fully canonical? Here's where my need for official, licenced opening titles are closing titles comes in. I consider them "basically canonical" - the lack of titles and such precludes full canonicity. The unfinished Doctor Who story "Shada" is "basically canonical" because it wasn't finished, but is commonly referenced and contains full titles. Average fans will vary widely in their knowledge of this material. Casual fans will probably not know much about them. If Nickelodeon did make clear, explicit advertisements in the primary initial medium of broadcast (in this case, television) to these webisodes and they had full titles, I would elevate them to canonical because average fans will be aware.
This makes a special like "A Fix with Sontarans" and such "more or less canonical." Similarly, parts of the iCarly promos that never make it into the final broadcast land in this spot. They are broadcast, casual fans will probably know them vaguely, but not have them on immediate recall.
What about other media? I put comic books, CDs, etc., as "sort of" canonical. That means that they are valid for analysis and discussion, but casual fans will not have a clue. I put Dan Schneider's notes, tweets, etc. into this group. I consider the canonicity of this material to be dubious because only the most intense fans will know about them.
Anything without some kind of official sanction is considered "non-canon." As good as the famous "Get a Life" sketch from SNL is, it can not be considered canonical.