Most people cannot come up with great answers to these questions on the spot. I recommend memorizing situations that can apply to these answers. Always keep several in your mind and be able to use them to fit the question. I keep a journal of interesting events that have happened to me while flying or working in general, before an interview I go over them and try to think about how they could be used to answer these types of questions.

It's not memorizing answers, but I am setting myself up to succeed. It has always worked for me, I'm never sweating trying to "come up with a time when...."