Here's an old video of a western riding class I competed in. This class is judged on the quality of the lead changes, and if they change leads exactly in the middle between the cones. There are some other things that go into it, but that's the main thing.
The main problem my horse had in this arena was the very tight space we had to work with (very small arena). Because of this, for my English horse I really had to work to make sure he was driving enough under himself to be able to change, with mixed results. It would have been easier if we had more space and could have had more forward impulsion. http://www.youtube.com/watch?v=0lG_xAZgvew
I'll try to talk you through the lead changes and say what mistakes were made, and what could have been done to correct it. Hopefully this will help you see what some of the problems are that you may encounter.
1st lead change - hard to see, but no obvious problem
2 - he was leaning on my right leg and refusing to get off of it. That's why it was so late. When this happens, (and it WILL happen with a green horse), you have to know how you are going to escalate your aids to get them OFF you're leg. For me, I start with my calf, then add spur, then pop them with my leg. If you have a crop, you might use it right at your heel. Whatever your method, know how you are going to do it. After I added my spur, the lead change itself was actually pretty nice.
3 and 4 - on these changes, he needed a little more forward impulsion (4 more than 3). You can see this by how he popped his front end up in the air, and seemed to get a bit stuck. So I needed to have him working off his hind end more and driving through - basically impulsion.
5 - on this change, you can see very easily how he does not stay straight; right as he changes, he drops that right shoulder out. I needed to block that with my reins, kind of an indirect rein on the right. Also could help correct by driving more with my left leg.
6 - this is one is pretty nice. He stays straight, flat over the topline, and forward
7 - he might have dropped his shoulder some, and also looked like he needed more impulsion
8 - pretty nice with no major problems.
But you can see how easily this horse guides and responds to cues, which is needed to get a nice change.
If you look at this video, right after the first jump (the x-bar), is a very very nice lead change. http://www.youtube.com/watch?v=FiDKUnFsDkQ&feature=user
You can see how easily he gets off my leg, stays flat, and both his front and hind leads change together. The main distance between this one and the other video is that he is moving forward. It makes a big difference with the horses, and makes it a lot easier for them, esp when they are learning.
One other thing to notice - I move him over BEFORE I ask the change (for a right to left, I move him over to the right and then change to left). However, it is almost impossible to see. With a horse that is learning, you would move them over more than this - but it is still not a big move; remember, it's just enough to get that weight shift.
I hope this helps. Let me know if you have any questions.