I am not by any means an expert, but here are my thoughts. Find something that interests you, either something on history channel or just wikipedia it. At the end of reading through, try to type up about a page focusing on as many different aspects of the subject as you can. I picked Genghis Khan as one, so I went through and wrote notes on religion, military strategy, overall philosophy, law, etc. You can easily make any topic fit into at least 3-4 of the 10 categories usually tested. Just by writing this out, you will remember more than enough details to fill 30 minutes of essay writing time. Keep in mind, as you do this, you want to be thinking about how you could contrast these points with something else.
The biggest take home lesson though is, don't make these prompt specific.
