This happens to me a lot. I'm not sure how to fix it, but I'd assume that just thinking about preventing your knees from caving in isn't going to actually fix it. There is probably a different problem or weakness that needs to be strengthen so that pressure is relieved from your knees.
Let me know if you know any more about this, I feel like it may just be from growing problems with knees, we both have that :P
I know my knees naturally bend inwards so to I believe to prevent it I have to strength my thighs and really any upper leg muscle. Especially when I get tired as I run I find my knees will occasionally touch each other.
Me too, and every now and then I'll trip because of it :/
One of the primary muscles for knee alignment (and specifically preventing the knee from collapsing in) is the sartorius which is a long muscle that starts on the outside of the hip and wraps around the front of the thigh to eventually insert into the tibia on the inside just below the knee. Strengthening the sartorius will obviously help but perhaps the most important thing is to train yourself to engage it to keep that alignment. If you don't have the neuromuscular connections to actual fire the muscle when you need it then it doesn't really matter how strong it is.
Look up sartorius strength training exercises online and see what you find. If you find some that look good and interesting to you try them for 3-4 weeks and see if you feel it's making a difference. Also, despite when Jan Erik says thinking about preventing your knees from caving in will help. Assuming you follow it up by forcing the knee out as you think about it while you run. That's the neuro-muscular connection training you are doing when you do that. Eventually that will become second nature and voila ;)