My big recommendation would be this guy, below - when we were walking round the Forum he was running a free tour which was absolutely fantastic. So good that we ended up booking and paying for a Vatican one, which was also really good. Not normally big on the guided tour thing, would prefer to research and discover stuff for myself but this guy really brought it to life: http://www.walksofitaly.com/about_us
Not much to add to that except to have bottomless pockets. Not as expensive as Venice though. Oh and don't bother with the commentary on the open-top bus tour, its rubbish.