Every so often, it's good to go over the basics once more.
A single-cog fixed gear offers four speeds to the rider: sitting, standing, weaving and walking. This applies most significantly to climbing, though sitting and standing apply to flat and rolling terrain as well.
Going down hill, riders can turn like a skier to help check speed. If you don't want to give up and use the brake, try cutting back and forth, if road width and traffic allow. So now you have both sitting and weaving available during descent.
Officially, most two-sided hubs have track threading on one side and freewheel threading on the other. Some riders choose to have a single-speed freewheel on one side of the hub. The technical term for these riders is "wussies."
Only kidding! But a freewheel does call for a brake or brakes.
I use a front brake on my fixed gear, just to have a little extra stopping power, and the ability to pull the front end down while braking into a corner. Without suspension, this effect is minuscule, but try grabbing a front brake alone, and grabbing a rear brake alone, to feel the difference for yourself. If you are an idiot, you will launch yourself over the bars when you grab the front brake alone, but let's assume you've learned enough by now to control the bike better than that.
If you slow by resistance-pedaling alone, the effect is not as smooth going into a corner. However, you can use well-timed resistance-pedaling to pop the bike up straighter for an instant at the apex of a turn. Fool around with it. You have nothing to lose but skin.
Round, smooth lines tend to work best, but you can have fun with some kinkier ones, too.
Returning to hub threading, a bottom bracket lock ring will thread onto most freewheel-diameter hub threads to hold a fixed cog in place on a road hub or the "wrong" side of a two-sided hub. Bottom bracket lock rings have just about gone with slotted cleats into the mists of oblivion, but cheap bikes still use them, and many are gathering on scrap piles. Millions must have been produced in the golden age of cup-and-cone. They come in different thicknesses, so you can find one to fit the space you need to fill. Using a BB lock ring I have fixed cogs on both sides of the rear wheel.
When I navigated urban streets I used narrow, 38 centimeter drop bars so I could shoot the gap between right-lane traffic and parked cars on skinny streets. Riding mostly on open roads now, I switched to 42-cm bars for more comfortable cruising.
Track bikes traditionally use nutted axles, but Tullio Campagnolo gave us the quick release axle for quicker wheel-flips during road races with fixed gears. It's perfectly legitimate, and highly convenient if you swap that wheel around much.
The Raleigh Grand Prix of the 1970s made a great basis for a fixed gear. For that matter, almost any modest road bike of the period had long dropouts and could be stripped down to a 20-ish-pound bike with little difficulty. With a bit more time and trouble you could refine the product even more. For instance, you should respace the rear axle and redish the wheel to straighten out the chain line. You might also invest in a track crank, though road cranks can fit a wider range of chain ring sizes. Choose your gears based on your riding area, and your own pedaling style.
The current VD epidemic in road bikes has limited their potential for conversion, but you can still find usable frames. And several manufacturers cater to the current fad for fixed gears by offering track bikes in their lineup. See Surly, Fuji and Soma for a start. For a year or two, everyone will be in on the act, until enough posers bust expensive dental work and the craze dies off. Gather ye componentry while ye may. Surly and Fuji seem truly committed to it. And look for sweet deals to come, when the riders who got bucked off dump their stash on eBay and craigslist.
Whenever a craze ends, as mountain biking did and fixed-gear riding inevitably will, it leaves a higher total number of riders than there were. So this moment of popularity will leave the fixed gear culture a little stronger. Fixed gear riders by definition ask little in the way of outside coddling and affirmation. Real cyclists are a self-reliant bunch, and career fixed-gear riders may be the crustiest of the lot.