1. This is the classic scenario: You're clipping along at a good pace, flicking through corners in a controlled rhythm, when around a blind bend you see water, dirt or some other debris directly in your path. What to do? Here, the rider has spotted the debris but is already committed to the cornering line, carrying a respectable amount of lean angle. This rider was able to spot the debris because he was looking well through the turn. Make sure you don't "ride the front wheel," which will limit your field of vision and therefore lessen the amount of time you have to react to certain situations. If it's water or dirt, it's not a good idea to cross it with very much lean angle. Once you've spotted the debris-but before you reach it-increase your lean angle to tighten your cornering line. This will give you more room to work later in the corner.
2. Just before you cross the debris, stand the bike straight up (or as close to it as possible). If necessary get on the brakes, but make sure to get your braking done early and release them before you get to the slick stuff.
3. Try to avoid braking through the problem area at all costs; it's immeasurably safer to roll through with the throttle slightly open than it is to even lightly apply the brakes.
4. Once past the offending slag, lean the bike back into the corner to avoid exiting your lane, which would either take you into oncoming traffic or off the road. It's a good idea to practice this procedure in an imaginary crisis when there is nothing at stake. Just remember: When the real thing happens, don't panic. Firm, thoughtful inputs will have you on your way without so much as a rise in heart rate.