Personally, I discourage my guests from buying Roma Pass as I see no much value in the pass. The link above lists all the Pros and Cons. Even after you read the same and still choose to buy the Roma Pass, here is how you can maximize the value from Roma Pass
Best way to use the 72 hour pass:
Day 1 - Vatican and Trastevere
If you are in Rome for the weekend for example, you can use your Roma pass to see all the main attractions. On the Saturday, you can head to the Vatican for the morning. The museums are not included in the pass, but you cannot do this on the Sunday. After this, you can head to the magnificent Castel Sant’Angelo and use your first free admission. You can use your unlimited access to free transport to explore the city for the evening, taking advantage of the nightlife in the Trastevere neighbourhood for example.
Day 2 - Colosseum, Forum, Palatine Hill, Borghese
On the Sunday, you can use your second free admission to see the Colosseum, Palatine Hill and the Roman Forum. You can use your pass to skip the lines at the Colosseum, then walk through the gate and across the road to Palatine Hill, then continue through the gate to the Forum. This will all count as one admission and it is a good way to maximize your access with the Roma Pass. In the afternoon you can use your pass to get a discounted entry to the beautiful Borghese Museum (but just be aware that you need to book a time to see this even if you have a Roma Pass). It would be worth using public transport to get from the Colosseum area to this museum. After this you can see free attractions like the surrounding Borghese gardens and then head to the Spanish Steps for the late afternoon and evening.
On your last day in Rome you can use the 72 hour pass to get onto public transport and see the outskirts of Rome like Appian Way, the less visited districts, and offbeat destinations. Check in advance for discounted entries using the Roma pass for any other ticketed attractions that you plan to visit.
If you choose to go with 48 hour version of Roma Pass, may be below suggestions will be relevant for you
Best way to use the 48 hour Roma pass:
Use the one free entry for Colosseum - Forum - Palatine Hill
You can still use your one free admission to see the three sites around the Colosseum, and a discounted entry to the stunning Borghese museum. These four attractions can be covered in one day, so for the rest of your time you could use your unlimited transport privileges to visit some of the lesser known churches and areas of Rome, and discover some of the free activities and more offbeat attractions around the city.
Let me tell you, I see no value in 48 hour Roma Pass