To be honest I found myself drifting upwards when I used to kick the kick shields often, but sparring quickly resolved this problem.
Originally Posted by Tenebrous
If I'm kicking upwards into someones ribs, or torso area I found that their elbow would connect with my instep and you only do that a few times because you quickly figure out how to come around with the kick correctly :)
Alive sparring FTW!
I specifically bought curved thai pads (though I prefer flat focus mitts & shields) because when holding them at an angle you wind up with some almost-flat surface for a turned-over-kick to hit, compared to flat pads that give you a purely straight slope.
I was always trained to lift the knee strait to the height I was kicking (higher for head) before turning over my hips with just flat kick shields held to the side. Where I always trained, for kick drills, pad holder would have two big 6" thick shields held in front of them, and the only time I felt it actually hurt was when holding pads for the 6'3 guy who outweighs me by 150 was kicking.
Though the holding at an angle we would do for practicing head kicks more with the upward kicking motion, because obviously they aren't going to hold the kick shields next to their heads.