Bianchi Pista: Stiff as fuck, how do I make it softer?

Which shoulder are you getting pain in? What kind of backpack are you using? It may not be the bike at all! :)

both side, near to where the collarbone is at, I have a small leather satchel with me.

Change the tyres - the Contis that come on it as standard are awful tyres.

I rather like it actually, just let me skid the hell out of the Contis on it before switching back to the Bontrager one, it's cheap but grippy as hell.