Try practicing with like 10 writing prompts in total, during your mcat prep. you will find that any other prompt you get, you will be able to cross-use your examples...so if you just develop a data bank in your head of like 10 examples, you are good to go for any writing prompt.
You just hve to use the example correctly and show why it supports the statement given.
p.s. always always always have a few political examples in your mind..cuz the chances of you getting a prompt about politics on the real thing is huge. I took the June 15th and I had one about social reform/leaders or something