When you have a forthcoming event, it's important that you start preparing your horse well in advance.
It largely depends on the season, of course! If it's winter, you may want to clip your horse, so depending on the stabling, work, etc. you can either do a full clip, a half clip or a trace clip. Make sure you have your horse clipped two weeks before the event. Again, ears being clipped out depends on the weather, your horse, and the work, but it is nice for them to be done, and then use a cloth to really clean the ears well. Get as much black wax out as possible, and make sure that ticks are not present.
The mane can either be hogged (not popular), or pulled. I don't like using scissors on a mane, as to me, there is nothing worse than the scissor look, but again, some horses are too sensitive to have their manes pulled. If you have to use scissors, then try to clip the ends, so it doesn't look do 'cut'. Depending on the breed of horse, clipping the legs to remove feathers, and hairs over the hoof gives the horse a more elegant look.
The tail can be pulled a little at the top, or really well washed and brushed. It should fall half way between the point of hock and the floor. Put your arm under the tail when measuring this to allow for the tail 'carriage'. If you have to give your horse a bath before the event, try to aim two days prior, so the horses coat does not stand on end. Manes and tails should be washed, if possible, and it's also nice to give the legs a good scrub, so the knees and hocks shine well on the day.
On the day, make sure that all the 'out of the way' places are brushed well, as there is nothing worse than noticing bedding stuck to the belly, or under the tail! The mane may be plaited, and white elastics used on a black mane are always more showy for the wrap around part. The top of the tail could be plaited, but personally, a well brushed, flowing tail looks good, and the horse will be more comfortable too. When the horse is thoroughly clean all over, then some baby oil around the muzzle and eyes is nice to show off the fine points. Hoof varnish, mainly black, can be applied, but make sure your horse stays on concrete till the varnish is dry, and also take care to be accurate at the top of the hoof, and apply varnish to the heel as well.