Here's what Rob Portman said about the last debate prep (in Vermont)...
Portman says Romney’s willingness to fully commit to the prep was striking. Day after day, he’d get up early, exercise, and then join the team for hours of work. Advisers certainly played a role, but according to Portman, it was the candidate who drove his advisers. Even when he had a busy week of campaigning, Romney would always find time to study or hold a brief mock debate. “It was all him,” Portman tells me. “Honestly, I’ve spent a lot of time with Mitt Romney for the past month or so, and what I saw on stage is who he is. He’s smart, he’s articulate, and he’s got a big heart.”Source: Romney’s Playbook
My understanding is that Portman is a good person to practice against. He studies the opposition’s candidate, attempts to use his mannerisms and really tries to get under the skin of the person he is training.