I have quite a few students riding the First Level 1 and 2 tests this month, and find myself saying the same things to everyone:
1. Make sure your halt is trot to halt - this isn't Training Level anymore, so no more walk steps.
2. If your horse halts and then throws his head down/up/sideways, don't try to correct the problem by moving your hands all over the place too. Practice keeping your hands low and forward of the saddle during the halt - just like in any other gait. If you properly halt from your seat aids, with your hands as a secondary aid, your horse will stay more correctly balanced into the halt.
3. If your horse's poll gets too low, pulling up with your hands will only put pressure on the bit in a way that makes him go deeper. Keep your hands soft and engage his hind end so that he develops more uphill balance.
4. Think of the lengthenings as a work in progress. You are not expected to snap into and out of an Olympic-quality extended trot yet. As long as you half halt to prepare an uphill, balanced lengthening and show a quality lengthening of the stride between the 2 quarterlines, then half halt when you hit the other track, to show a clear transition down to the trot you had before the diagonal, you have done well. Overly abrupt transitions will only lead to loss of balance.
5. Lengthen the canter more than you planned, as long as you can maintain control. Start your downward transition back at the second to last letter of the long side, no later. Again, your horse isn't balanced enough yet for overly abrupt transitions in such a powerful pace.
6. Start your leg yields earlier than you think you need to. Take your time getting over. If you rush sideways, you will lose balance. Try to get to the quarterline halfway down your leg yield - no sooner or later, so you are getting the job done but not rushing.
7. Did I mention maintain a good uphill balance into a soft contact? Rinse and repeat.
Good luck in your test :)