I strength train my legs. I believe this prevents injury, and also makes you a more efficient runner. The last time I was injured was partly due to me cutting way back on strength training in favor of running more (at least in my opinion, this was part of the cause).
When I run, I do speedwork Monday, Easy Tuesday, Long Thursday, and Easy Saturday. Following the Easy runs on Tuesday and Saturday are when I work my legs. Wednesday, Friday, and Sunday are when I work the core and upper body (legs rest for the most part).
My main leg exercises are:
Wall squats - ball behind the back and another between the knees while doing these. Hits the VMO muscle well.
Single leg squats
Hip Hikes
Calf raises - single leg with an eccentric drop.
Bridges
Heel walking
Single leg balancing exercises
Those are my main exercises. I usually add a couple others from time to time including:
Single leg dead lifts
Leg extensions
Leg curls
Lunges (be careful with these, make sure the form is on. I have actually injured myself doing these before).