---------------
Good resources to get you up to speed on teaching this topic include:
Crystal's Encyclopaedia of the English Language esp the - first
seven chapters.
a couple of chapters of Graddol et al, History of the English Language: history, diversity and change.
Freeborn's From Old English to Standard English: because it is a fantastic source of "old" texts.
ed. adds : I agree - I've done a handout using some of the facsimile texts from Swift, Johnson, Lowth and Cobbet that are in here - it is great for getting a grip on the whole standards debate, and for reading some "original" texts ... we spent quite a while on this, then linked it up to a few bits and pieces on the emergence fo the National Curriculum in English - (including a great quotation from Tebbit where he more or less says that having "poor" grammar makes you a criminal!)
for a more audio-visaul angle - - look at Bragg's Routes of English series see http://www.bbc.co.uk/radio4/routesofenglish/
- also available on CD-ROM.
ed adds: and linked to quite a bit of "activity" on the website if you follow that link