If you have problems with carrying your hands/arms... think of having your elbows are the heaviest part of your arm, like you've got weights attached to them. I found after riding lots of green horses, my elbows were coming away from my sides quite a lot, so this one really helped me ;)
Also with your hands, there's a lot of 'white knuckled' riders out there. Imagine you're holding two small birds in your hands instead of reins. THAT is the amount of pressure you want to have. If you grip too tightly you will squash them, too loose and they will fly away.
To test how still you keep your hands, put a neckstrap on your horse and hold it while you trot. You'll feel yourself wanting to pull on the neckstrap as you trot if you're used to moving your hands around.
If you ride with your hands uneven, ride with a short crop or stick under your thumbs, horizontally, and try to keep it level while you ride.
When rise trotting, put your leg on when you sit. You will have the most influence over the horse's hind legs at this point.
When cantering, canter from your 'core' not your pelvis. Yes it is your pelvis that moves with the horse, but how many of us look like we're trying to shine our saddle with our bottom while cantering? :P So keep that backside firmly in the saddle by moving your core muscles and holding them with the horse's movement, and sinking into the saddle.
I have so many but I couldn't be bothered putting them all up at once :P