…eat Croissants for breakfast, and eat Gelato every day. Life will be good.
Also, carry a trustworthy guide book. It will be your friend. Seriously, I always have guidebooks when I travel, but I rarely carry them walking around. Rome is different. Not because it's harder to navigate (it's not), but because it's easier to get caught eating in tourist traps of dubitable quality, where a guidebook can help you find better options for local fare in the neighborhood you find yourself. You’re already going to pay a bunch -- Rome is très expensive -- so you might as well get quality. Seriously worthwhile. We did much better with the ever-helpful Lonely Planet than when we chose on our own.
Be prepared to cover your shoulders and knees to enter the Vatican museum. Same thing goes for St. Peter’s (if you brave the lines, which we didn’t).
If it’s summer, hydrate early and often. And keep in mind that public bathrooms may be scarce, and/or icky, but still better than dehydration.
Leave time for window shopping when walking anywhere.