In reply to colina: Allow me to give some advice from my professional position as a podiatrist.
Frontal knee problems are most commonly associated with excessive pronation of the forefoot, most commonly associated with a rearfoot varus, anatomical fixed position forefoot or occasionally a limb length difference (not an unusual occurance). Such issues of movement create compansatory movement elsewhere in the foot and lower limb, even including the torso - for every movement in one direction there is a compensatory movement in the opposite direction.
When you walk, or run, downhil this creates increased impact/intensity across the forefoot as the impact is greater there if you have reduced movement at the heel etc and you will be doing more forefoot loading. Such a knee pain is also commonly associated with back pain across the lip of the pelvis.
Some of the issues that cause this problem are quite often not easily rectified by one thing but many things collectively can help and even resolve it.
Stretching of the ankle, knee and hip are clear in that they help movement to occur and compensatory movement be reduced. But this isn't the only solution. Strengthening exercises, specifically of the quads, also helps desist that impact.
Orthoses may help or be needed in the long run for some but these are not always the panacea for all foot problems. To help as a simple orthoses, try a heel raise (felt or sorbathane may do it) but don't make it more than 5mm thick. This brings the heel into play more effectively and again reduces the movement.
If this doesn't help I would always advgise seeing a sports-injury podiatrist who can give you advice on all of these facets as well as assess open and closed chain motion (non-weight bearing and weight bearing)of the foot to see what other help can be provided.