The most important thing you can do is to set your intention to accept this training and the attunements with an open heart and mind.
Some people have found it helpful to stay away from sugar, caffeine, alcohol, red meat, and recreational substances at least 3-7 days prior to the class. If you feel this is an appropriate path for you, by all means do what feels right.
Meditation, rest, and spending time in nature are all very good ways to center and connect to your Self before and after the training.
If you do not have prior experience with Reiki, it is highly recommended that you receive a full hour session prior to the training in order to familiarize yourself with this modality.