Ummm.... re the advice... well considering how I have this problem (STILL) I don't really have anything that works, but if you make sure your thumbs are on top (coffee mug hands - you're carrying too mugs of boiling hot coffee, where are your thumbs going to be?) that usually naturally brings your hands up. But yes, it is a nasty habit to break!! I'm getting better about it but if Monty gets too forward, BAM down go the hands again.
Hands in the correct position means softer hands but also more leverage to say STOP RIGHT NOW if you need to. My naughty hands have a habit of going back and down whenever I'm riding a circle, particularly my inside hand (both ways), which does not help when Monty wants to pop his shoulder on the circle to the right.
Rob any kind of english riding requires carried hands/forearms and it's a very common habit to have them too low. So a lot of people actually need to lift their hands!
If and only if your horse will tolerate it, take 2 dressage whips and hold them as if they were reins. Let your arms hang like wet towels so your elbows are at your sides relaxed. Without changing their position, hold the whips so they line up with your horse's bit. That's the line you're looking for. Just please make sure your horse can handle two whips next to her face before you do this!
Hold your reins the correct English way, make sure thumb is on top. Put a little bit of bend in your thumb and think of it as the "roof" to your hand. So, keep the rooftop thumb on top where it belongs, AND the thumbnail should point as keen as a laser directly at the bit ring, no matter where you put your hands, but really laser in with the thumb tip and make sure that the laser comes all the way from your elbow, throught the rooftop thumbs, straight to the bit rings, which you can imagine yourself holding in your hands, fingers wrapped around them.
All instructors are different, i'm always being told to keep my hands lower than I do, If you have your hands lower its a lot easier to get your horse on the bit, But they should'' be resting on the horse, and your hands shouldn't be too close together. As for distance keep them at the same width as your horse shoulders (our if its a big house the inner shoulder) and as for height keep the bottom of you hand level with the top of your pummel.