Sacktavius has a good point. It's all well and good to have the billets you are comfortable with, but I think mentalism really shines in situations where it seems improvised. It's very much a magician's mindset to practice the physical movements of switching a billet, and making sure you have a specific paper cut to a specific size to do things. However, as Sacktavius points out, mentalism relies far more on the performance skills. Being confident and bold will get you very far. My fiancee is pretty well versed in magic and mentalism at this point and even she doesn't see the switches I use because I use them at the appropriate time.
Instead of focusing on the physical properties I would recommend focusing on learning to direct attention away from the hands entirely so that you can do almost anything to switch ... well, almost anything.
The HP Lovecraft of Bizarre Magic