How can I select suitable leaves for hammered leaf art?
My wife and I did a bit of this the last time we were in Japan. It was very difficult, but also very interesting. 

As the previous answer indicated, experimentation is key here. In fact, I almost think it's the whole point of this technique. Each time you fail miserably, you'll learn a bit more about what doesn't work. 

Try everything you come across (it's all free materials anyway). Make notes on what did/didn't work. You can number your papers to correlate with your notes.

You may find that different plant materials respond better under different hammering techniques.

Once you have developed a feel for it (through experience), you will quickly develop your own signature style.

Don't be afraid to fail miserably.