TYPICAL ERROR I have really enjoyed watching football on television this year. Especially as this is the first time we have seen it on a 3D screen.
CORRECTION I have really enjoyed watching football on television this year - especially as this is the first time we have seen it on a 3D screen.
EXPLANATION In the example above, the words in the second ‘sentence’ cannot be punctuated as a separate sentence because they do not make sense on their own. They are an afterthought, and so a dash is the most appropriate punctuation to use.
Here is an example of how to use a dash to introduce an afterthought:
As you can see, dashes tend to signal an abrupt change in the direction of the stream of thought - not that this style should be used very often, mind you. It is best to use dashes very sparingly, especially in formal writing. If you find yourself using dashes frequently, your style is probably lazy - and perhaps disjointed.