Knee Popping when Standing?

I recently starting working at a clothing store where I have to stand on my feet for about 6 hours a day. I noticed my right knee started hurting, and now it pops and hurts whenever I move it after standing on it for only about an hour or so. Because of my job, I am required to wear clothing from the store and tennis shoes are not allowed in my dress code so wearing shoes with better support isn't much of an option. About 8 years ago I injured this knee in a skiing accident but I never had any problems with it before so I don't know if this is related. Is there any way I can prevent my knee from popping or keep it from hurting so much? Do you think a knee brace would help? thanks!