If you're doing local and not USEA sanctioned, I'd just do rosettes for Dressage, and then let it hang naturally for cross country and Show jumping. That's what I've done at Beginner Novice with my Quarter. The one show I was doing, I wasn't going to braid but the barn owner yelled at me that morning to braid it, it wasn't pulled and it looked like crap!
If you do the diamond braid, just button/rosette braid the forelock.
I once saw a lady who was growing out her horses mane, it was too long for the regular braids but too short for a full running braid, so the did a series of short running braids with nice tucked up ends. It was beautiful, unique, and looked very professional.
Heres a really really bad drawing that kinda gives an idea: braids.jpg
They were much smaller and more refined on her horse though...I fail at necks
I think what masatisan is referring to is the scalloped braid... look it up to see it in photos and how to do it.
I used to think a running braid couldn't possibly work for the thinner long manes either, but with a little persistence, I've perfected the technique. You've got to add very small sections of hair at a time, so that there are more "links" if that makes any sense. You also have to stay as high and tight against the horse's neck as you can. I highly suggest standing on a stool or bucket. If you do it right, your hands will be cramped by the end. ;D
I recently did a running braid on my instructor's thin-maned Thoroughbred. Here's what it turned out like:
Nono, what I posted is not scallops, those are like...glorified maintenance braids or something. What I'm talking about is a running braid that is broken into smaller braids so that if the hair is shorter, it wont fall out. Also the breaks make the braids more flexible and less prone to falling, especially if the horse is moving a lot.
I tried to do it on my practicing tail (its supposed to be a tail, not a mane, so it didnt work out the best): Picture 591.jpg
Kinda bad pictures too sorry about that...but from the side you can kinda see. Picture 592.jpg