Jay - to agree and disagree with some of the comments: Yes, you can buy one-ride tickets and multi-day passes at most news-stands or tabacchi. Also a few coffee bars will sell them. You can easily get a one-ride ticket but occasionally they are out of the passes. I have usually bought the month passes because of the lengths of my trips, but I always validate the pass on every ride. There is an Italian habit of attempting to ride free by not buying a ticket (same in Florence) and I don't want the driver or others to think I'm a cheat. Unless the bus is very crowded it's easy to stick the ticket or pass into the validation machine, usually just past the driver or soon thereafter. VERY IMPORTANT whether you validate or not: keep the receipt with you for any passes you buy if the exact usage dates are not written on the pass. Not sure about the multi-day passes, but the month pass has no markings that give the dates or even name of the month. Therefore, if you get checked (happened to me more than once) you'll need to show the receipt with purchase date so the checker knows it's valid. In Florence, when you swipe a pass, it automatically deducts the ride and shows you how many rides remain.
As for entering and exiting any Rome or Florence bus, you enter by the driver and you exit by the MIDDLE door further back. Otherwise people will get p-o'd because you're blocking their ability to get on the bus, especially if crowds are waiting to get on. If there are crowds, be prepared to NOT be able to get on, from either door. The driver will let as many people as possible pack into the bus, as long as the doors can be closed. It's kind of amusing, unless you're in a hurry: the driver keeps trying to close the doors but cannot so he tries again while riders are scrunching even closer together until one of two things happens - either he's finally able to close the doors or several people step backwards off the bus so the doors will close. Worst case, you just catch the next bus.
Be aware that at some of the busier stops there are quite a few bus lines that stop at different positions, so look up at the signs to make sure you are standing near the sign for the bus number you want. It's always a good idea to wave at the driver (like hailing a cab) when the bus you want is approaching; that way he knows to stop for sure since someone wants to get on. Drivers will always stop at the busiest/most crowded stops, but if you're at a non-crowded place with just a couple bus lines passing buy, unless you wave the bus will keep on going because the driver assumes you want a different bus. Also be aware that the step down from a Rome bus to the street is high (same in Florence) so watch your step so you don't land in a pothole, of which there are many!
On last thing, it's not constant, but not uncommon, that the bus you expect to arrive just won't show up. As you probably know, when you use Google Maps for directions from A to B, it will give you the bus numbers and the times they arrive. However Google does not have intimate knowledge of Rome's government and financial dysfunctions, i.e., between corruption and lack of money, buses break down while you're riding (the driver will tell everyone to get off) and broken buses take forever (or never) get repaired.
Don't let any of this dissuade you! It is a unique part of the VERY glorious city of Rome. And if you have interesting bus adventures (or misadventures) these will make great stories to tell when you get home! Happy travels!!
EDIT: - no, you don't show your pass to the drivers; they're too busy trying to understand (with difficulty) questions from American tourists and making sure they don't run over anyone when they start moving
