Weight lifting, sprints, jogging, and power walking. Weight lifting is great for the leg muscles and helps maintain two point. I find parallel squats to be the most helpful. Sprints are great cardio; run full out for 50 yards, catch your breath, and run back. I do this around 10 times. Jogging is nice for endurance, as is power walking. Also, running up stairs is good for cardio and leg muscle.
Stretches are important. I stretch every night to keep myself flexible and supple. It minimizes aching in the saddle and keeps you from pulling muscles.
What kind of diet do you maintain?
I eat what I want; I just try to eat small amounts of it. A slice of cake is fine if it's just a slice, etc. I try to stay away from sugars and red meat, but otherwise my diet isn't very controlled.
Any special vitamins or routines?
I walk every night and stretch every night. This is important, because if I don't do something constantly, I fall out of habit. I do sprints at school during PE and lift whenever I have access to free weights.
I also ride daily, weather permiting. 25% walk, 50% trot, 25% canter. Sometimes with less canter and more walk, depending on how my horse and I feel that day.