Universal Sports has a piece on Michael Phelps at NBAC and how he did. They also discuss what he goes through before he enters the water for warm-up and a snippet of what sort of warm-up he does. From Universal Sports:
"... Lying on his back, Phelps was massaged and stretched by the North Baltimore Aquatic Club's training staff on the third and final day of the NBAC Christmas Meet. [...]
...they returned about four hours later to prepare for the finals session. Phelps laid down on the training table and got stretched out. He occupied himself with his BlackBerry as he body was pushed and prodded. [...]
He eventually jumped in the warm-up pool at about 5:35 p.m., a few minutes after the first final of the night -- [...]
He lazily swam laps in lane 1, going from an easy freestyle stroke to employing a kickboard as he geared up for the 200y butterfly later in the evening. [...] ..."
Be sure to go visit, there is an awesome photo gallery of the event and the best photos by far are his interactions with the kids there. Click on the picture of Phelps which is below: "Related To This Article Photos"