1. Never taking it easy. As a runner when your off day is an hour of biking or swimming and then a little lifting and your off season is less than a week you are flirting with burnout. Many very dedicated athletes train 40 hours a week but have a serious off season where their cortisol or adrenaline or whatever really gets a chance to rest.
2. School or work or personal life drama consume you. Attending a difficult engineering school caused my friends and I to stress out about important projects several times and performances to just plummet.
3. Ramping up training too fast. Injuries happen this way too. One week you're running 30 miles a week, the next it's 50, and then 70 and then you get injured or feel tired all the time.
So train hard and take it easy!