I ride every day, but I still need to keep fitter for the level I am doing. I find that doing cardio, full body strength and core work are what help me the most.
Right now my routine per week is 50 minutes of spin (indoor cycling), 30 minutes of regular yoga or stretching, 60 minutes of active hot yoga, 60 minutes of circuit training with cardio, weights (upper and lower body) and ab work on top of riding 1 horse every day. During the summer I cycle every day and during winter I'm going to start cross country skiing as well once or twice a week. I like being fit if you can't tell!! I believe that maintaining a high level of fitness is the number one way to improve your riding and prevent injury!
They say money doesn't buy happiness -- well happiness doesn't buy horses!