There's not much to be said about countersteering over what you can find all over the web. It is the most important way to steer and control a bike.
If you didn't hear about it - it is the rather counter-intuitive way you steer a motorcycle or bicycle, by steering initially in the opposite direction. To test this, get on your bike and get it going strait with some speed. Then, just push the left handlebar and observe what happens next. Make sure you are not compensating by pushing the handlebars or moving your body in any other way - just observe what happens: the bike will become unsettled and lean over to the left and starts a left turn! It is counter-intuitive because you basically directed the wheel to the right and the bike turns left!
Without going into a religious debate about it, the best explanation I have for counter-steering is that moving the front tire contact patch out of alignment (say to the right) will "throw" the bike over the other way, starting the turn to the left. This is why it is more pronounced and more obvious at higher speeds... the inertia of 300kg moving at 80 km/h trumps any other forces that may be at play.
Other ways to control the bike, that I know:
How to practice countersteering? That's easy. Find an open road with no traffic and try to go between the dashed lines on the pavement - faster and faster, by pushing on the inside handle only. Start at say 40 kmh and work your way up - I can do it up to 70 kmh after which I go for two at a time.
As you go faster, you will have to also pull the outside handle to pick the bike up faster - it's actually pretty cool when you feel and understand that racing term...
You should practice some steering and braking combinations as well.
Countsteering also works to adjust your line in the middle of the turn. Just push the inside handlebar to lean the bike more and or the outside bar to reduce the lean angle and turning radius. The reason is that it works by upsetting the equilibrium, either going strait or already turning.
There is a point in calling it push-steering instead.
If you're still reading at this point, check this out: Skiing and countersteering++.