Jen,
The story of 5-hr lines at the coliseum is an urban legend. This year the Forum, Palatine Hill, and the Coliseum are all bundled on one ticket. You can purchase the tickets specifically for those locations at each of their entry points - and use on your first day and THROUGH 1:30 PM the next day. (It's not exactly a 2-day period as some vendors advertise... )
The shortest lines if you Purchase the three-pack tickets are at the Palatine Hill entrance on Via Di San Gregorio. The tickets are €11. Once you have these tickets when you go to the coliseum entrance, get into the shortest security line (there are two metal detectors, thus two lines - the left line usually moves quicker as that's where groups head!).
Once you clear the metal detectors, GO LEFT - as far left as you can go and walk on the outside of any line you see. You'll come to signage that says Groups or Pre-Purchased tickets --- continue to GO LEFT through this area until you reach the turn-style where you ENTER the Coliseum. I just did this last week with David and his family from Rochester, MN and we walked right in - NO WAITING once we passed security!
You can also purchase a Roma Pass for €20 (a great deal if you're going to Villa Borghese which is now €13.50). Follow the same entry steps and slide your Roma Pass into the turn-style and in you go!
Ciao,
Ron