1) Don't order water from restaurants! If your plan to save money is to drink water -rather than wine- when out...you should know that a glass of water is no cheaper than a glass of wine. Instead, utilize the drinking fountains! Rome is known for having some of the purest tap water around. You can find a drinking fountain on almost every corner, which has water constantly flowing out of the side. You stick a finger in the side hole and the water than comes out of the top, at a perfect height for you to drink out of. My recommendation would be to keep one water bottle and keep refilling it at the free-of-charge fountains.
2) A bus/metro ticket, good for 75 minutes of unlimited bus and metro travel, costs one euro. It is the law that as soon as you hop on your first bus, you must validate this ticket in the validation machine. While it is the law to buy a ticket and validate it yourself...it should be known that I have never actually bought nor validated a ticket to use the bus in Rome, and have never gotten fined. That being said...unless you are staying near Termini Station or going out clubbing in Testaccio, there is really no reason to use public transportation. If you are staying in the city center, everything is within a mile walk from your hotel. If you are on the outer-center, the farthest you will walk is a mile and a half. And with the countless unique shops and sights, walking is the best choice in my opinion.
3) Pre-gaming: Unless you are a girl, willing to do bodyshots or show a Drunken Ship bartender your boobs, drinks at bars and clubs can get expensive. However, there are always little cafes and random hole-in-the-wall shops that offer one euro shots of certain liquors, like Jager. Take advantage of these, or else you'll be paying 5 times as much at the pub next door.
Here are some cheap eats in Rome:
Outdoor market in Campo di Fiori- Fresh fruits and vegetables...great for breakfast where you can buy enough fruit to fill yourself up on for under 3 euros.
-Aristocampo, a panini cafe with locations in both Trastevere, by Santa Maria de la Scala, and also in Campo di Fiori, is basically the Subway of Rome. With dozens of different paninis and also a build your own panini, most of which are under 2.50 euro.
-Carlomenta is a great place for a real Roman dinner, off of Viali di Trastevere, where you can get an entire pizza Margherita, and glass for wine for under 6 euro. There are also quite a few pasta dishes, such as gnocchi and carbonara, under 5 euro.
-Blue Ice, which can be found on almost every corner in Rome, is like the Baskin Robbins of Rome. You can get gelato for 2 euro, as well as delicious Nutella crepes, and they are usually open into the late hours of the night.
-There are also little pizza cafes that serve pizza by the slice, for 2 euros, which is great for a quick lunch or a snack as you explore the charming neighborhoods of Rome.
Try not to eat in the ultra-touristy areas, as you'll pay double, if not more, than you should for a meal. Anywhere along Via del Corso is going to be almost three times as expensive as a non-tourist area. Also, keep in mind that most restaurants will add on a service charge to your bill. This is considered the tip. You can leave a few euros extra if you'd like, but it's not expected.
As for sight seeing; try to plan your trip around the last Sunday of the month, when you can enter the Vatican for free. When it comes to the Colosseum, most people say that going inside is a waste. Don't pay for a tour of anything. View it all on your own.
To me, the best part about Rome is the beauty and charm of the city. It costs nothing to take in the beauty, so wander the streets, it's free! I guarantee you won't be bored by the character of Rome!
Speaking of it's beauty and Charm, I shall leave you with some of my favorite photos of the city.
