If you're going with the Mrs i recommend going to the Trastevere area at night, really nice restaurants and bars, nice winding cobbled streets etc. My favourite area when i went.
If you're going to the vatican, Colosseum etc get all your tickets now before you go as the queues are mental. Also, plan to eat about 10 times a day! The food is too good to just eat 2 or three times!
^^ very good advice...Tickets in advance for the Vatican as people have said, you won't regret it. Make a note of the major attractions and get around them but apart from that, just wander round. So many things to see just around the corner.
Wonderful place. I went in Sept and it was bloody hot. August is supposed to be uncomfortably warm as you will do a lot of wanking.