if you think u can go into the exam having a generic essay on lear.. you will get done over.. king lear, you cant realli "prepare" and essay, more so u can have quality notes on aspects of the play.. like u said.. going through themes/values, language use (commanding, imagery), content, productions, QUOTES, values of a certain scene, maybe even know 2 extracts, as in 2004 they asked for you to refer to two extracts...
make note on textual integrity and its reception in a range of contexts...
Make sure you have your own interpretation also, whether it be taking bits and pieces out of other "critics/readings" to develop your own one.. just have one incase and back this up with quotes,scenes content and themes...
hope that helps...