There are a few good reasons for giving your tomato plants some support, and many, many ways to do so.
Staking tomatoes in some way gives you cleaner fruits and healthier plants. You will keep the tomatoes away from slugs and mud, and the improved air circulation that results helps prevent disease.
Some tomato varieties though actually produce better if left to sprawl. Most of the plum tomatoes fall into this category. I’ve found for instance, that my favorite drying tomato – Principe Borghese, does best unsupported, while the Super Marzano are too heavy and prolific to leave sprawling on the ground.
I’ll place some straw under those that I do allow to roam free, to keep the fruits clean and dry.
You’re probably all familiar with the old standby conical wire cage. Of course, nowadays they come in bright colors, too. This is my least favorite method and I never use them.
The biggest disadvantage I see is that the ripe tomatoes can often be difficult to see, and to pick deep within the cage. They also can fall over if the plants get heavy.
One advantage to using cages of some kind is that the plants do not need to be pruned or tied (more on that later). I prefer this homemade wooden cage – you can purchase kits for making them if you don’t want to do it yourself. (If you would like to make some, here are instructions.)
Other methods work best if the vines are pruned to one or two main stems. The simplest method is your basic stake – nothing more than a post stuck in the ground.
The post could be almost anything – a piece of pipe, a T-post or a simple piece of wood. A one-by-two with a pointed end is a good, inexpensive option. Whatever you use should be long enough for the plant’s eventual height, and strong enough to hold the weight of the tomatoes. Add nine to twelve inches to the expected height of the plant to allow for what will be underground.
You can also purchase tall steel supports known as curly stakes. They now come in decorative colors in addition to the original green, which look cool. Choose them for looks. They work OK, but I’ve found that you still need to assist the plant, at least at the beginning. I didn’t need to tie this one, but I am still training it to climb the stake. 
By far my favorite method though, is a hog panel (at least until I saw my neighbor’s trellis, built from discarded display racks!). It’s basically a 4×16 foot panel of heavy, welded wire. Now I know that you city dwellers probably can’t easily get a hold of one of these, but I’m sure there are other options that would work the same way.
The tomatoes are pruned to one branch, then simply woven through the wires as they grow. when the growing tip reaches a horizontal bar, I push the leaves through to the other side, which eventually pulls the stem with them.
