Playing him in an advanced role is essential, as some of you have said. The deep-lying playmaker role was reinvented using Pirlo at the start of the decade, but I'd prefer if he wasn't restrained by defensive duties in favour of a more dynamic, attacking role. Not necessarily behind two strikers, but at least freed from the holding role in front of the defense. Ambro's passing skills are underrated I feel, he's more than capable of sitting in front of the back four and starting attacks (not necessarily "spraying passes" a la Andrea, but the key to shutting out our midfield has become far too predictable for the opponents).
That's not a knock on Pirlo's defensive skills, they're quite good and if he had the strength and pace of Essien, I would rate him as the most complete midfielder in Europe. He doesn't, sadly, so he must be utilised properly - on the left, right or top of the diamond in our 4-3-1-2, where his attacking qualities will be of more use without worrying about the defense. Prime example is the Catania game, arguably his worst of the season. Fielded alongside Seedorf and Boateng, whose natural attacking instincts saw Pirlo - on one occasion - chasing Maxi Lopez as the second last man. Things like that should be avoided, although Allegri would be stupid not to realise his mistake in that game, especially as he kept bitching about how we're susceptible to counters.
I don't want an Inter-like negative midfield of three defensive-minded players, fuck that. At the moment, however, it's delivering results so the most sensible change would be trying out Pirlo as a trequartista. Although in the long-term, I'd like to see a return to a more flexible midfield; Pirlo, Boateng and two defensive midfielders can work even better, as getting the right workrate is more important than the personnel. Any role that leaves Pirlo free of defensive duties where he's far more prone to losing the ball and sparking a dangerous counter, and allows him to put his creativity to use (and I'll say it again, his technical qualities are inferior to none - not Xavi, not Schweinsteiger or whoever...).
One criticism is that I get the feeling that he lives in his own fantasy world at times, which isn't that far off-the-mark if his facial expressions are anything to go by. Pirlo goes for the spectacular on some occasions when a simple pass to keep the rhythm and possession going could do, but he sees a perfect pass in his head and expects other players to latch on to them. Teammates don't always understand the runs he wants them to be making.