When getting your knee down, you are pushing both your bike and yourself further than you usually would.  Don't forget, there are several factors that could cause you to come a cropper, so for goodness sake be safe when sliding.  Common sense prevails when choosing knee down locations.  Obviously, on the track, the main considerations are tyre pressures and temperatures.  However, some riders choose to slide on the road and this is potentially far more risky.  Knee down should only be attempted on track, however if any of your friends decide to try on the road, at least check the following:

• Check your bike's condition.  Leaky forks or spilling oil/coolant will not only pose a problem for you, but other riders as well.  Ensure your suspension and brakes are in good order.
• Check your tyre pressures and general tyre condition.  You do NOT want a blowout whilst getting your knee down
• Check the road surface.  Mud, disesel and many other things can contribute to your bike parting company with the tarmac.  You don't want any of these things present as 100% concentration is required.  Obviously, don't attempt knee down on cold roads or in the winter!
  
• Check the rider!  If you're tired, hungry, upset etc. don't risk it.  Go home and try another day.  You have to be 100% concentrated and anything that detracts from that is a bad thing!
• Make sure the tarmac is warm and WARM YOUR TYRES UP FIRST!  Just a quick trip around the block or up a dual carriageway and back should get enough heat in your tyres.  Just make sure they're warm and sticky so they grip properly.