I put on my armor...
This is a really neat concept! I've got "relaxing" clothes just for this very same purpose.
I believe you can get your mind ready for Aikido the same way you would get your mind ready for anything else: an exam, a work shift, a writing session, a workout, and so forth. It's different for each person.
For me, I do a combination of breath/visualization exercises with the purpose of phasing everything else out of my picture except what I'm about to do. I become what I'm going to do, keeping a mindful awareness of the present, and try to let everything else fade into the background. If something arises in my mind, I simply acknowledge it, let it go, and return to my breath and the sounds/sensations around me.
The here-and-now is a great place to start!