What is the proper state of mind or concentration - kavvanah - that one must have when reciting Jewish prayers?

We wrote about this subject in a paper, "Kavvanah (Concentration) for Prayer in the Mishnah and Talmud."

...Rabbinic sources express sophisticated notions regarding inner states of consciousness. When we examine several rabbinic texts and translate into more contemporary terms some concepts of the rabbinic rules and interpretations regarding inner states of mind, we discover strikingly mature attitudes towards those aspects of consciousness, intention or concentration during prayer, called in the texts, "kavvanah for prayer." ...historical analysis of the concept of kavvanah in early rabbinic sources shows that the idea does not remain static within rabbinic thought but evolves in the various documents...

You can read our essay here.