Last couple of weeks I started having knee pain and was wondering if it could be due to improper foot placement or technique. Going to the bike shop today to buy clips and bike shoes (maybe that will help), at least I will be more efficient. Bring it on!
In theory cycling puts less strain on the knees than does walking, so I expect that your problem is either one of technique or fit. Sheldon Brown goes into cycling pain more fully than I can here.http://www.sheldonbrown.com/pain.html
The most likely cause is that you are trying to ride in too high a gear. The aim is to have the pedals going around at 90 rpm, at that speed very little pressure should be needed to keep you moving.
Another likely reason is that your saddle is too low. When the pedal is at the bottom of its stroke your leg should just have a slight bend. If it's more than that you are not getting full benefit from the down stroke and then lifting the knee really high on the up stroke. Your local bike shop (LBS) should be able to check this and adjust if necessary.
Another possible reason is that you are cycling with your knees pointing outwards. Your hips, knees, feet should all be in a straight line. Sheldon gives a better description of this than I can.
If you have stiff knees and long cranks then this could be another reason. However, this is a less likely reason than the others.
Firstly try using lower gears, get your saddle height checked and keep a nice straight line between hips, knees and pedals.
I hope this helps.