Ride to the markers exactly so, for example, when you change the rein across a diagonal you will ride a deep corner. Also make sure that your circles (and any other similar movements in your test) are as accurate as possible. Try to keep your horse active as well (moving forwards), and try and keep relaxed so that your horse doesnt tense up which will affect their paces. Remember that walk is the most important pace too, as its usually worth 10 X 2 marks in a dressage test, and dont forget to practice your free walk on a long (not loose) rein, where your horse should still have an active walk but their head should reach down close to the floor. Also make sure that you show a difference between medium and working (if there is any in the test(s) you are going to do. If you can, get your pony round and deep, but remember that you will get marked down if your horse is behind the bit as well as if they are infront of the bit. Try to make sure your pony is as fit as possible as well because stiffness will loose you some marks.  Remember to bend when you should and keep your horse dead straight on the centre line (practice riding the centre line so you know if your pony tends to drift and make sure that you know you can get them to halt square and straight at the end of the test). Finally, make all your transitions as smooth as possible, and remember that around 50% is a good score for your first dressage competition, so don't be worried if your score seems low at first. :)