Healthy joints despite heavy exercise

Hi guys!

I currently do a lot of exercise and most of it is "heavy" ie roller blading, running, elliptical trainer. I was just wondering what ideas/info people have about maintaining your knee and hip joints. I currently take gluocosamine (sp) supps and also fish oil which is meant to help with joint mobility but wondered what else there is.

Some days if I do a 30min run in the morn and walk home an hour from work followed by similar exercise the next day my knees really ache. I'm only 22! And I'm only 70kgs so I'm not so overweight that my fat would be stressing out my joints. I do mix it up with pilates and swimming, I try to swim as often as I can.

THanks!