For the clipping - I don't generally clip my horse. But he doesn't grow much of a coat and it's very cold here. It is all up to you. But if you do clip her make sure that if you bath her before the show and after clipping, then add some baby oil to your rinse water and her coat will be shinier.
For the braiding - I recommend braiding, but not the hunter/jumper braids. Generally in the dressage ring, I only braid in between 9 and 15 braids. To do this I use braiding elastics, a comb and a damp sponge. I divide the mane into sections and restrain them with the bands, making sure to crisply divide each even section. When you are happy with your sections, then start braiding. Wet down one section at a time and braid them as tightly as possible, finishing with an elastic band. When you are finished braiding all the sections then fold or roll them up depending on how long your horse's mane is and secure them with another elastic band. Done! For the forelock, you can either french braid it if you and your horse are patient, or bring some gel and smooth the hair down and braid it like you did the mane.
Here's a picture of how my braids end up: http://i46.photobucket.com/albums/f1...s/IMG_8019.jpg
Also, in case you're wondering about what to wear, that's my horse and me turned out for a small schooling show in my area. I highly suggest white saddle pad, white breeches and if possible a show jacket, but a white shirt and crisp vest is also acceptable.