Antarctica is a land of extremes: it is the coldest and driest continent on Earth and has the highest average elevation. As the fifth largest continent in the world, Antartica is also the most Southern, overlying the "South Pole".
The most common method of visiting the Antarctic is by Antarctica Cruise ship. In the Antarctic summer, several companies offer excursions on ice strengthened vessels to Antarctica. Ice strengthened (not quite as tough as icebreakers) ships are preferred since icebreakers are round on the bottom - a configuration that amplifies the already massive wave action in the Drake passage.
The ships typically offer a couple of excursions to the continent (usually the Antarctic peninsula) or Antarctic islands (e.g., Deception Island, Aitcho Island) each day over the course of a week. The views are phenomenal, the penguins are friendly (well, some of them are), and the experience is one that is unparalleled!
When travelling by boat, be aware that smaller ships (typically carrying 50-100 passengers) can go where the big ships can't, getting you up closer to Antarctica's nature and wildlife. Larger vessels (carrying as many as 1200 people) are less prone to rough seas but have more limited landing options. Many vessels include naturalist guided hikes, zodiac excursions and sea kayaking right from the ship, perfect for active, casual travellers.
You'll need warm clothing: boots, hoods, glove, water repellent pants, parka and warm underwear. Most of these items can be bought or hired in Ushuaia, but sometimes - in the high season - it is not always easy to get the right sizes. So bring whatever you can from your own stock.
It must also be remembered that cruise operators typically only allow 100 people on land at any one time in order to comply with IAATO agreements. Consequently if you are in a boat with more than 200 people the chances are you will only spend a couple of hours at most per day off ship. Generally the smaller ships will try to ensure 2 different locations per day around Antarctica, although this is of course dependent on the weather and you may expect a 60% success rate on landing people for any given visit.
Many shipping companies are now offering Fly/Cruise options, which entails a one-way or round-trip flight from either Santiago or Punta Arenas to King George Island. These are often pricier than typically cruises that cross the Drake Passage both ways, but cut 1-3 days off the total travel time.