was just there in July..hot as hell but should be way cooler now....
go to the coliseum when it's about to close. you avoid the long ass lines and it stays open past their closing time. plus it's nicer to take pictures of the coliseum instead of a coliseum filled to the rim with tourists.
and if you visit the vatican...do a tour. you DO NOT want to wait in the regular admission line. it wraps around blocks...doing a tour gets you straight in and it's actually more informative.
oh..and eat a gripload of gelato. it's the best. haha.
Lol, my advice is just about the exact opposite. I was there in September, weather wasn't too bad, nice cool mornings but the afternoons got a little warm.
We went to the Coliseum first thing in the morning, like 8AM since our sleep patterns hadn't caught up and we were waking up at the ass crack of dawn. No real line whatsoever by the time it opened at about 8:45.
St. Peters Basilica... hit it early before it fills up with (too many) tourists, it's free.
The Vatican Museum, like arcarc said will have a line around two whole blocks or more. I almost got suckered into the tour, I tried haggling, cause the tours are around 40 euro on top of normal entry, but no takers. When we got out and grabbed some lunch at a trattoria right across the street after going through the whole things, the two block line had completely vanished by the time we finished lunch. If you hit the museum around 2 in the afternoon there shouldn't be a wait.
The Spanish Steps and Trevi Fountain were both crowded as hell all day long so it doesn't really matter if you go morning, noon, or night. And be ready to be accosted by all sorts of scammy little fucks trying to offer to take your picture or give your girl a flower for "free".
Most of our money was spent at various restaurants and bars, the tipping culture isn't the same as in the states. 1 or 2 euro is enough of a tip at restaurants but they'll always hit you with a bread charge.
Watch out for cabbies, especially when leaving the airport, don't let a gypsy cab grab your stuff, go to the official queue, and even then you have to watch the cabbie as he will try and grab money out of your hand while you try and get the right denomination together. The best way is to have 30 or 40 euro separate from the rest of your money for cabbies in case they get "grabby".
The food is great for the most part.
Rome is pretty busy, I liked Florence a little more, it was definitely more laid back. But both were great and if you're a history buff you're going to love it.