Ok aside from the obvious DONT CRASH! there are some simple steps to take that will ensure that your knees dont explode after long periods of cycling and yes that does include Singlespeed
1. Seat height: a general rule of thumb that I always follow and found to be pretty accurate for comfort and least pressure on the knee is to place your heel on a pedal on the most downward position of the crank (whilst sat on the bike obviously!). Your leg should almost be straight not out-stretched. Here are some great seat-height setting methods to try
2. Saddle position: this may well be personal preference and will relate to the size of frame you have chosen, but I have found that getting the right combination of seatpost type (i.e. inline or layback) and setting the saddle in the correct horizontal position can make a huge difference. I aim to seat myself as close to vertically above the crank as possible which has given me immediate relief from the iliotibial band syndrome (ITBS) I was suffering over recent endurance races, There is a raft of seat positioning videos on youtube
3. Go clipless: dont let the stigma of embarrassing crashes put you off trying clipless pedals, if you have wandering feet they can help you maintain the correct foot position as you pedal and provide a more positive drive failing that make sure you wear some sticky trainers to help on flat pedals
4. Glucosamine: take it daily – its not a sign of age, its lube for your joints in the same way you would lube youre chain, and you dont want to be replacing them like an abused, stretched chain now do you?!
5. Warming up / warming down: even just some simple stretches can be enough to get some warmth into those knees and warming down can be just as important here is some great stretch information
Enjoy many more years of pain-free cycling, pay attention to what your body is telling you if you get a twinge in a knee then maybe its time to change the way you are riding, and the above steps are a good starting point. For more information on knee pain check out http://www.bigkneepain.com/