The trick is training! I just managed to get Lyra to do this for me:
I'm so proud of my girl! She started off as "just a pet", so naturally, I taught her to "sit" before she got anything (food, toys, go out, etc.).. but since I decided to show her (she was always a show quality girl--just has a minor underbite that we're hoping will correct soon!).. I had to get her to stop sitting when I didn't tell her to. Most show puppies are never taught "sit" simply because it hinders their ability to stack and stand nicely at all times in the show ring.
So this is what I did to teach Lyra not to sit when she smells food (it took a while--she absolutely LOVES food and kept sitting everytime I put her on the table!): I taught her the "wait" command (as opposed to "stay", which means sit and stay. "Wait" means stand still). I would say "wait" and give her a treat if she stood still for even 1 second. Slowly I moved up to 2 secs, 3 secs, etc. Now she can go about a minute without moving when I say "wait". What I do is, I say "wait", and put the treat right in front of her nose, a little bit up, because I want her to stretch her neck a little (that's how you stack a dog).
Lots of training is required! I'm currently getting friends and family who come and visit to "act" as a judge.. by checking Lyra's teeth, touching her body and such. If she stands still and is a good girl, she gets a treat.
About structure, look at Lyra's picture above. Look at her topline--the straighter it is, the better. Neck length is another thing. Longer is better (obviously not too long, but too short is no good). And look at her rear legs. Judges look at "angulation" while looking at the rear legs.
Here's a picture of what good structure should look like:
http://www.weim.net/emberweims/images/WeimAngles.jpg
See the angulation? You want the dog's rear legs to look like that when stacked. A bad angulation looks like this:
(That's a picture of my Papillon, Cadence, who is not a show quality dog. See how his rear legs are very straight as opposed to angled?)
I said that Lyra's rear legs should be further back in my picture because that stack doesn't show her angulation well, and because her legs are not further back, it made her topline look weird (it's not straight, as it should be). Also, judges look at the front of the dog.. You see them touching the dog's chest? They're checking to see if their fronts are good--some dogs have very shallow (narrow) chests and that's bad. Also, they look at the front legs. They have to be straight with the paws facing straight. Here's a good comparison:
See how Lyra's legs and paws are straight while Cadence's paws turn out? (He was sitting weird, his paws are actually not that badly turned out, lol).
Anyway, I like explaining things to people--I myself have only learned all this within the past year, so I'm still getting there! Ask away if you have any more questions. Oh, one more thing.. the judges look at the dogs' gaits while they are walked around the ring--you want a dog who had good reach (meaning they take big "steps"). A dog who is well-built and balanced will have great reach. Other things judges look for are showmanship, temperament, etc. A dog whose tail is down or tucked between its legs all the time is bad--Cavaliers are supposed to be friendly and outgoing, so they must never growl or snap at a judge. Also, tail set is quite important. The highest a Cavalier's tail should naturally be is about 1 o'clock. It should never be higher than that (this is what people call a "gay tail").
Regarding Forrest's spot on his face--it could be a hindrance if you're wanting to show him. Some judges will fault that, but I've seen one or two champion Cavaliers who had brown on the side of their faces like Forrest does.