When plaiting the mane, try to keep it as tight to the neck as possible so that it's easier to roll up. Use hairspray to keep it in place. I always use at least two bands per plait if I'm going to a show - those bands have a habit of snapping the second the the judge pays attention to you!
If your horse has a particularly long (or just scraggly) mane, use a Native plait instead of those little balls - they look way lush!
For tails, instead of plaiting the whole way down, use a series of bands and fold the end under the main plait so that you can't tell it hasn't been plaited to the end - it saves a lot of time and still looks great!