Man.. this could not have come at a better time as I am still experiencing some knee pains. Anyhoo, while perusing taiji literature, I ran across the following passage:
7. The knees
Avoid downward pressing of the knees. Imagine the knee is always directed upwards when squatting down, pushing forward or forming a horse stance.
– Source: Neigong.net – The Quintessence of Wu (Yuxiang) Style Taijiquan
In my own training, I have been intently focusing on sinking down into the soles of my feet. I imagine the tension in my body going down into the ground. As such, when I do feel some strain in my knees, I imagine that tension going down. I guess, in a way, I am willing the pain to go down into the ground. However, after reading the above passage, my mental intent is most likely resulting in a downward pressing of the knees!
So, in doing some zhan zhuang, I imagined my knees directing up. Whadda ya know.. less strain. I tried some side to side chen style shifting, less strain. I formed a bow stance and imagine the knee pushing forward, less strain. The strain is lessening to the degree that I barely feel any strain or tension in the knees.
Amazing what a little bit of intention can do.. 
