DRESSAGE - Maximise your potential with a good warm up

The importance of warming up your horse before entering the arena.

Your aim to a successful warm-up is to be at your peak just as you go in to do your test. However there are several other factors that need to be taken into account, such as your journey to the event, the weather, excitability and timings of your class, these can all have an impact on your performance and meticulous planning can help settle pre-competition nerves and enable you to be mentally focused on your riding and execution of the test.
You should allow plenty of time to arrive, get your bearings and check the running order to see if they are running to time or are running late. If they are running late be sure to ask if the judge's will be taking their breaks. There have been occasions when judges have not taken breaks to catch-up and this has resulted in riders thinking they have more time than they actually do. As a rider you do not have to go before your allotted time but if another competitor has withdrawn or they are running early then you can if you want to go in before your set time.
Mark Butler_-_Blitz_at_Hickstead
If you have had a long journey make sure you allow plenty of time to walk your horse or pony on arrival in case they are stiff, a purposeful 10 minute walk is really beneficial in helping a horse to loosen up and you know yourself how stiff you can feel after a long journey!

Very warm weather can zap energy levels of both horse and rider; you will know your horse or pony and will be able to judge how the heat might affect them. Generally speaking you can usually do a little bit less with most horses if it is very hot to avoid overcooking them and ending up with a flat or tired horse.

If you have a horse that gets very lit up at competitions you may find a double warm-up helps. By getting on first several hours ahead of your test and working in you can help eliminate any high jinks and ensure that when you get back on to warm-up for your test your horse is more focused on you having already taken in the sights and sounds on your initial warm-up.
Your warm-up should aim to bring you and your horse to your peak just before you are due to complete your test, you do not want to try anything 'new' instead you should stick to a clear and established routine, following exercises and patterns that allow you to work through any issues and helping you to supple your horse, get them working over their back and developing connection.
You will have a clear idea of the exercises and patterns that suit your horse so ensure that you have a pre-planned warm-up routine in your mind. This should be a basic drill that you can run through to enable you to 'warm up' your horse. As you work through each pace, you will get a feel of how your horse is and in turn you should be able to fine tune and adjust the exercises to help you work through any issues or problems you might encounter. 
For example a horse who is feeling flat or on it's shoulder would benefit from spiralling in and out on a circle through leg yield, both in trot and canter to encourage them to take the weight onto their hind legs and so liberating the forehand. Alternatively a horse who is unsettled would benefit from walk, trot, walk, transitions to focus the horse on the rider.
Always be aware of other competitors who will be working in around you, while you want to maximize the use of the warm-up you do not want to commandeer it, so think ahead and plan where you will ride.
Remember to take your horse's boots off and to drop your whip before you enter at A if it is a championship and finally smile! You go there on merit so enjoy it and good luck!