Pompeii is a wonderful place to visit, a kind of dream trip especially when combined with other destinations in Italy. I have had the good fortune to see them multiple times as I worked on my book Bodies from the Ash: Life and Death in Ancient Pompeii. Based on my travels there, I have these recommendations:
Traveling to Italy. The best (and least expensive) time to visit Pompeii is from November through April. Airfares are much more affordable, hotels drop their rates, and (most important) fewer busloads of tourists crowd the narrow streets of Pompeii. It's true that the weather can be more variable then, but you can also have splendid weather in the middle of December and January. It's worth the risk if money is a factor.
Choosing a place to stay. To visit Pompeii, you should plan to arrive shortly before the ruins open at 8:30 which means that it is most convenient to stay somewhere nearby.
The key is to spend at least one full day at Pompeii, which means that you should plan on spending the night in the vicinity so that you can arrive at the ticket office by 8:30 in the morning (the posted opening time).
Staying in Naples or Sorrento: You also stay in Naples or Sorrento (both are appealing bases). Sorrento is somewhat quieter and more sedate; Naples is bustling and lively.
How to arrive by car: If you drive to Pompeii, there are many parking lots across from the main entrance (Porta Marina) that charge about €5 a day. You cannot miss these small, private lots, since lot employees standing near the street will wave you in.
How to arrive by train: On the other hand, it is very easy to hop on the Circumvesuviana train to get to Pompeii from either Sorrento or Naples. There are two Circumvesuviana stations in Pompeii, each serving a different line of the train:
(1) If you come to Pompeii from Sorrento or if you take the Sorrento line from Naples, your stop will be the Villa of the Mysteries station (Pompeii Scavi), which is directly across the street from Porta Marina, the main tourist entrance to the archaeological site.
(2) If you come from Naples on the Poggiomarino line, your stop will be the Pompeii City station, which is located more in the center of modern Pompeii. In this case, you will walk straight out of the train station until you reach the cathedral square (you can't miss the imposing spire). Then turn right and walk along the main street until you reach the Piazza Anfiteatro, a much quieter entrance to the ruins (my preference). If you are at this entrance when the ruins open, you can avoid the crowds for an hour or so as you explore the eastern ruins.
Train fares are low and very reasonable (for example a daily ticket from Naples to Pompeii costs about €5.00 and includes unlimited metro rides as well).
Entering the ruins. For me, there is only one way to see Pompeii. Arrive shortly before the opening time of 8:30 a.m. outside the Piazza Anfiteatro entrance. This is far away from almost all of the tourist buses (which arrive at the Porta Marina entrance). When the ticket seller shows up (often after the designated opening time), you will still be one of the few people waiting to enter the ruins. Using the excellent map that's provided free for visitors, you can stroll into the uncrowded southern part of the ruins through the Nucerian Gate. Then head for the Garden of the Fugitives and then walk as far east as you can...to the amphitheater and any of the streets along its north side. For the better part of an hour, you may be able to feel as if you are in Pompeii alone.
A slight digression. To make the most of any visit to Pompeii, it is wise to do some reading first. Many people hold a number of misconceptions about the eruption in AD 79 (no, there wasn't any lava flow) and the eventual discovery of the ruins. There are no museum-like information placards posted in the ruins, so unless you are knowledgeable about Pompeii, your visit may not be very meaningful (one ruined building after another). I have provided some suggestions in the sidebar on the right, plus more choices for supplementary reading. And if you are interested in the plaster casts, I recommend my own book, Bodies from the Ash, which provides a thorough discussion of the casts, both how they were made and how archaeologists have deduced information about Pompeian life from them.
Getting organized for a visit. These days it costs around €11.00 to see the ruins. Unless you are a member of the EU, there is no discount for seniors or students. Still, a non-EU family of four can visit Pompeii for less than half the price of an adult ticket to Disneyland...not a bad deal to my mind. My only complaint is that once you enter, you cannot leave without paying again (there are no hand stamps). This means that you should be very organized:
1. Bring a large bottle of water (if the day will be warm) and perhaps something to eat. A cafe is located north of the Forum (along with a small bookshop) in case you want to eat in the ruins.
2. Take a guidebook. Even if you have educated yourself about the ruins, a good guidebook is invaluable. Vendors sell them on the way into the ruins, but these are mostly just cheap tourist guides. You would be better served to plan ahead and buy one before you arrive at the site. I recommend this guidebook. However, even if you forget to bring a guidebook, you will find a good shop at the Porta Marina entrance, which sells a complete range of guidebooks (and various souvenirs) in all major languages.
3. Wear comfortable shoes. The main streets in Pompeii are treacherous with their undulating stones, and I have seen more than one person fall.
4. Plan to go back another day. One visit just whets your appetite for more. If I could, I would visit Pompeii every day--it is that fascinating a place.
5. And of course, bring a good camera, because you will want to take many photos and/or movies. It helps if your camera works well in low light (without a flash) since many rooms are dimly lit. Be prepared for lots of tourists in your shots, unless you arrive early in the morning.
A Special Ticket: If you wish to visit both Pompeii and Herculaneum, you can buy one ticket (valid for three days) that allows entry to these two sites as well as Oplontis, Stabiae, and Boscoreale (the adult price of about €20.00 is a slight bargain if you plan to visit two sites, but a huge value if you will visit all five; note you can only visit each site once during the three days).
Eating a meal in Pompeii. There is only one place to eat in ancient Pompeii. Fortunately, the tired, old cafeteria has been replaced by a fresh new Autogrill (the same chain you will see on the autostradas). The food may be ordinary, but it is a cut above the former fare (and thank you, Rick Bauer, for the following photos).
You will enjoy your food more if you plan to eat outside the site in modern Pompeii, which is filled with tourist restaurants and one McDonalds (between the cathedral and the Piazza Anfiteatro entrance); I recommend that you avoid these. One moderately priced restaurant is the Carlo Alberti restaurant on a street of the same name, just off the cathedral square. The pizza and pasta dishes are excellent. If you are on a budget, there is an adjoining storefront where you can order pizza to go; almost all pizzas were around €5 and worth every euro cent.