Motorbike in good working order. It does not have to be the latest race rep. I can comfortably get my knee down on my '82 GSX1100 streetfighter.
Good tyres - this is a must!
Knee sliders (First time I got my knee down was with leathers on, but no slider. It hurt and left a hole in my leathers and my knee - not recommended!)
Where
The best place is on a race track, but many people also use roundabouts or motorway slipway roads. Any long corner, over 120 degrees is good, because you spend a reasonable time at max lean.
The corner must have a good surface free of potholes, manhole covers etc. NOTE; I do not know the legality of getting your knee down on a public road, but I doubt that the Police would be too happy if they see you. The best place is on the track.
The ideal corner is a 3rd gear corner. This means you will be traveling at a reasonable rate, but not too fast. 1st or 2nd gear corners are a bit tight and the bike tends to be snatchy on and off the throttle which can upset the motorbike.
If you you choose to use a roundabout pick a quiet one that gives you good visibility of traffic joining the roundabout.
Preparation
Make sure you are feeling comfortable on the motorbike and you have done a few miles to warm the bike and tyres up.
Ensure the corner you are going to use is free of grit, diesel spills etc, you want to get your knee down not your arse! Best way to do this is to get off your motorbike and check. If not ride round a few times - this also helps warm the tyres.
Go For It!
Approach the corner at normal speed. Well before you get to the peel in point slide your arse over the seat get at least one cheek off, but better still get the whole of your arse off the seat so your outside leg is across the saddle. But only get off this far if you can keep your foot on the peg. Grip the tank with your outer leg and arm.
Then stick knee out a bit. The balls of your feet should be on pegs, but they should be there for all riding anyway.
You must move your whole body towards the inside of the motorbike, not just your arse. Try not to twist your body.
Turn into the corner until you are leant right over. It is unlikely you will get your knee down straight away. So while lent over lean the bike down a little more. You can genitally lean down a bit more and then up again, a bit like racers do when warming tyres, except you are lent over.
Didn't work? If not have another go. This time hanging off more. It is generally easier to hang off the motorbike more than lean it more. At first the amount you need to hang off will feel a bit odd. You do not need to be getting the bike over to anywhere near the edge of the tyres to get your knee down.
Still can not do it? Well there are plenty of fast riders who can not. Shelley pictured above races in the National Endurance championship, she can not get her knee down, but she can lap a race track faster than meanly everyone in the fast group on track days.
You can always buy some used sliders and tell your mates you managed it!
My Get Out
This is advice from my experience. If you try following this advice you do so at your own risk.
Have a look at the MCN video below for more advice.
Motorbike in good working order. It does not have to be the latest race rep. I can comfortably get my knee down on my '82 GSX1100 streetfighter.
Turn into the corner until you are leant right over. It is unlikely you will get your knee down straight away. So while lent over lean the bike down a little more. You can genitally lean down a bit more and then up again, a bit like racers do when warming tyres, except you are lent over. 
