Warming up your engine
One of the more enduring automotive myths is that you have to let a vehicle idle to warm up for a while. This is the worst way you can get your motorcycle (or car) to the proper operating temperature. Your bike should be under a load (moving) to get it warmed up well, and also to extend its life.
There is zero harm to starting a cold engine and putting it under a LIGHT load right away. On a bike this means choke it, start it up, and then as soon as you can give it throttle without dying (usually just several seconds) ride away. This does not mean bash the hell out of it as soon as you're out of the driveway.
Close the choke (enrichener) as soon as you can. There are lots of good reasons for this, but just do it. You may have to turn the choke down in increments during the first couple minutes of your ride before the bike will run fine without the choke. This can often mean having it applied halfway for the first few minutes, but if the bike is in a good state of tune you shouldn't need more than a few minutes before you can turn it off for good. Just remember that it is a manual choke and you have to remember to shut it off.
Why shouldn't I let it warm up while I'm putting on my gear?
Engines warm drastically faster when under load than when under no load (idling). The evils of idling are ultra low oil pressure at a time when the oil is at its thickest and doesn't flow very well. That means that right after starting is the point where you have the worst lubrication. By riding you're forcing the oil to move more and provide lubrication to the points that aren't bathed in oil (think heads). Using a good synthetic oil with a low 'cold' number, such as Shell Rotella T 5w-40, will mitigate this as much as possible.
The engine also needs a higher concentration of fuel to air at startup, as the atomization of the gasoline is poor in a cold engine. This means that the sides of the cylinders are getting hosed down with gas, which washes off the little lubrication that was there (and isn't being replaced at idle, due to the factors discussed above).
The last, and most important, issue is that you create localized hot spots by idling. The engine does not warm evenly. When a mass of metal is bolted together, as an internal combustion engine is, it heats rapidly in some places and not in others. This makes it more susceptible to warping and seizing/galling, which is where two metal surfaces stick/weld together, then rip apart, leaving both surfaces weakened.
So, take it easy until you can see that the engine is warmed up; then you can ride like your normal squidly self.