Someone had the right idea by talking about speed -- the speed your foot/leg is travelling directly effects the power of your clearance, or any other kick for that matter. Of course, form and many other aspects come into play as well.
To improve leg strength, do some research. And don't do the ankle weights, they put unnatural pressure on your joints. If you look at the training programs for great athletes around the world you will see several constants in their workouts. Examples would be Squats, and Deadlifts, two great excercises that promote muscle growth, particularly in the legs.
Calves have a VERY minor role in kicking power. More important would be thighs, hamstrings, hip flexors, glutes, and your abdominals. If you don't want to pump some iron, look into plyometrics -- they can, of course, be included with a lifting routine as well.
Please, the great majority of the people on these boards would do themselves a world of good if they took the time to actually look into this stuff. The amount of useless garbage I see spread around is sad. There is no one "right way" to do it, but your results will come quicker if you know what your doing rather then listening to everything you here proliferated around here.
As far as actually clearing it, yea, just lean back and drive it. It isn't a cross, its a clear, I don't know why you would be worried about lofting it up field. Deliver it to somewhere your forwards can play and away from the other team and let them do their job.
