Keuring Prep: I think there are many ways to prepare. For those lucky enough to have a trainer or be able to send theirhorse out for prep- you are lucky. I live out here in the middle of no where with very few Friesians, so I did all my "prep" on my own. I read books, watched videos, attended other keurings before mine and... trail rode my horse! Yep, Rode in the mountains!
My first suggestion, don't be in a hurry! Since judges are getting tougher, make sure your horse "looks" mature They all mature differently,and I know everyone wants to go at 3, but you can also go at 4 or so for the first time too. Consider waiting a year if your horse still looks like a baby. I went with my mare at 4, and I am SO glad I did. I believe had I gone at 3, she would not have gotten ster. There was a HUGE difference in her at 3 and 4 yo.
Also, as for mares, make sure they are developed and muscled, not overly bulky or anything, but in shape. More than once I have heard the Dutch judges say that we don't "work" our mares. I think they want to see our mare as much in shape as our geldings and stallions. My advice, if your horse is ridable- ride it! Best exercise there is! And consider carefully a decision to try a mare for ster when she has a foal on her. Some will do well, if trained, exercised and fit, but so often baby sucks so much out of mama, that she doesn't look good.
Movement- I think the trot is very important. The walk is too, the definitely look at that, but a short weak walk (what they said about my mare) can be overcome by a long, powerful and high action trot! I agree that the horse's natural gait is going to come to the surface first and foremost. You can definitely improve it. And believe it or not, the person showing the horse (runner, owner, who ever...) can make a difference. You want the horse relaxed enough that they have a natural walk, but at the same time excited enough to "mooove!" If you can run (or have someone who can) I think practicing running your horse to the music you are going to use also helps. They associate the music with showing off and some horses will just perform big time!
And for "Mom" and "Dad"- relax! Have fun!