In a post called "Deposition Tips for the Young Lawyers Out There," the anonymous author notes that in his first few depositions, he wasn't exactly sure what to do about witnesses who couldn't recall the details about the subject of the deposition. He advises now to take things "step by step"--
My first couple deposition were very hard for me. The witnesses always seemed to have “forgotten” details about the accident. Now I know that this is what is supposed to happen in a deposition, but with the little experience that I had at the time, I just couldn’t figure out how to get around the dreaded, “I cannot recall.” It seemed that no matter what I said, the witness couldn’t remember a thing. Later on and with more practice, I found that laying a foundation and taking it one step at a time really helped.
The post doesn't suggest particular questions to ask, but most lawyers will agree that in initially getting the witness's story out, the question "And what happened next?" works very well. After the outline emerges, you can go back and pin down the details with more specifically-worded questions.