Priority number one should be mastering an efficient running form. Build gradually and be patient with it. That will save your legs from the majority of common running injuries. Routine strength training and comprehensive stretching should cover nearly everything else from a prevention standpoint. From a recovery standpoint, proper nutrition, compression, and ICE! Oh yeah, and no pushing through injuries. They're a sign that you already pushed too much.

I've taken Glucosamine and Chondroiten (sp) since my early 40's. It helps to keep the knee joint lubricated.
Change your running shoes often and learn to get the "feel" for when they need to be replaced. The 300-500 miles rule doesn't work for some folks.
Maintain a lifting program at least part of the year that keeps the muscles around the knee strong...thus keep the knee stable.
If you run trails, develop good footwork and pay attention. A moments inattention can tear a knee apart.
Yes, trail running is better because of the softer surface but injuries tend to be more catastrophic.
Don't mash a big gear all the time.
Avoid ball sports.
Take an "off" day once a week.
Keep your fingers crossed that you have good genetics. Some folks are born with thinner cartilage than others.

this suggestion is perhaps too late...but i'll throw it out there anyway for next year...

Use your off season as a run focus. go short and do it more often, 5 or 6 days (with easy days built in). This will strengthen your body to take the pounding.

Now that you're in your tri training...I'd suggest making sure you are warming up and down. Take the first and last mile of your tempo and long runs super easy. Stretch regularly.