Musically, it is important to be aware of what you are doing that helps to create that emotion. What notes are you emphasizing? Where are your crescendos, decrescendos, accelerations and ritards? What dynamics are you using and where? If you do not know how you want all this to be shaped, it will be a crap shoot whether or not you will get the expression you want from your piece. Once you have all that mapped out, you need to practice those things in addition to the notes and rhythms. Make sure you are bringing out the expression you want every time.

The more you practice and the more you know your piece so well you can execute it the same way over and over, the more comfortable you will be with it, the better chance you have of doing it right.

There is an additional step to being in "the zone", though, and in my experience that is related to being relaxed. I have done things to practice relaxing. I do something I call "zen scales" and play my scales super slowly and softly, focusing on making sure my posture is right, my shoulders are down, fingers are curved appropriately, and everything is just so relaxed I almost sink into the keys. It is almost a type of meditation, but it is highly directed. I become very aware of what that state feels like in my body. Now, when I say to myself "relax", my muscles know exactly what to do and I can recreate that feeling. (I do not do my zen scales before performance. They are something I do every so often during practice to help me "practice relaxing." It does help later, when during performance or right before, I tell myself to relax.)

You may also want to read the book, "The Inner Game of Music" which also goes into detail about visualization.